Irrationality of the dilogarithm at
Abstract.
We prove that is irrational. Irrationality of was known for integers and by work of Hata and of Rhin and Viola, and Prellberg recently proved it for . We define polynomials with rational coefficients by an interpolation determinant. The rank of this determinant modulo primes with bounds the denominators of the coefficients. A concavity inequality for a logarithmic energy, combined with the inversion , bounds . Interval arithmetic is used only for the energy and the potentials of a printed reference measure, on a uniform dyadic subdivision. The integral of the local rank function is a rational sum over printed affine pieces. The coefficients of and the determinant modulo are printed and can be checked by hand. A congruence modulo shows that for every rational and , large in terms of .
Key words and phrases:
dilogarithm, irrationality, interpolation determinant, logarithmic energy, interval arithmetic2020 Mathematics Subject Classification:
Primary 11J72; Secondary 33B301. Introduction
For let . Hata [5, Theorem 1.1] proved that is irrational for every integer and every . Rhin and Viola settled by their permutation group method [10, Section 6], and later treated negative arguments [11, Theorem 2.1 and Section 4]. Viola and Zudilin [13] introduced a six-parameter family of double integrals for dilogarithmic values. In 2024 Calegari, Dimitrov and Tang [2, Remark 2.8.2 and Section 15.2] listed the integers still open: . Prellberg’s 2026 preprint [9, Theorem 1.1] proves the cases . We prove the case . Our linear forms are values of an interpolation determinant of size , in place of the double integrals of [5, 9, 10, 13]. The rank of this determinant modulo primes controls the denominators of its coefficients.
Theorem 1.
The number is irrational.
By Landen’s identity [8, (25.12.3)], , and is transcendental by the Hermite–Lindemann theorem, a special case of [1, Theorem 1.4]. So Landen’s identity does not reduce the case to the case .
Put and . Section 2 defines by an interpolation determinant. Sections 3 and 4 give
| (1.1) |
The exact value of is in (3.12), and is a lower bound for the constant of (4.5). The finite constant of Lemma 4.4 depends only on the reference measure. Since , the quadratic exponent is negative. If with , then . Section 5 shows that it is nonzero for and large, and (1.1) shows that it tends to zero.
The only numerical margin in the proof is . The size of is not used. Lemma 4.2 is computer-assisted. It is proved by interval arithmetic on the reference measure of Tables C.1 and C.2, over the uniform subdivision of Lemma C.1. The integral (3.8) is an exact sum over the affine pieces of Table A.2, each determined by the explicit lines of Table A. Tables 2.1, A and A.2 and (5.2) can be checked by hand. Python scripts recheck Lemma 4.2 and part of the remaining proof (Appendix D). Apart from this lemma, the proof is complete without them.
The proof uses three classical inputs.
Fact 1 (prime number theorem [4, Theorems 6 and 420, §§1.8 and 22.4]).
satisfies as .
Only this qualitative form is used.
Fact 2 ([12, Lemma I.1.8]).
If is a real compactly supported signed measure of total mass zero with , then .
A short proof is given in Appendix E.
A proof, including the sign, is given in the proof of Lemma 2.2.
The use of AI tools is described at the end of the paper.
2. The determinant
Put . For let
all ordered increasingly. For put
For , and let
and put
| (2.1) |
a determinant of size . For an indeterminate the same formulas define
Lemma 2.1.
One has , , and
| (2.2) |
Proof.
Write and
Polynomial division and the geometric series give
The substitution gives
Hence the first identity in (2.2). Add times each row to the corresponding row of . Only the middle rows depend on , and their coefficients are rational. ∎
Put and . Let be the segment from to , and let
a linear combination of paths with complex coefficients. For every integer ,
| (2.3) |
For integrate on each path. For the segment gives and the circle gives . The total variation is
| (2.4) |
With as in Fact 3, use variables , and .
Lemma 2.2.
With product differentials ,
| (2.5) |
where
Proof.
By (2.3), . Put . Then
Set and . Multilinearity in the rows and antisymmetrization, applied to the rows , then to the rows , and then to the functions , give
where
The last equality follows from the Cauchy–Vandermonde identity. Explicitly, for ,
and , . Fact 3 and this row scaling give the formula for . Continuity covers .
We prove Fact 3 as an identity of rational functions, with independent indeterminates. We use induction on , for all at once. The case is the Vandermonde identity. Let , let , and let denote with omitted. As a function of , expansion along the last column writes the determinant as plus a polynomial of degree at most , where is the minor without row and column . The right side is a polynomial of degree in divided by , so it has the same form. By the induction hypothesis for , is the right side of the fact for . We have , and at . Hence, after the factors and cancel, the residue of the right side at is
So the residues agree, and the difference of the two sides is a polynomial in of degree at most , zero if . It vanishes at the points , , where the determinant has two equal columns and . Hence it is zero.
The product of the four factors is . Since , is even and . All integrals converge absolutely: on , on , and has finite variation. ∎
3. The divisor
Put
For let . From now on is a running index, and the parameter of Section 2 equals . Fix the seam . Primes are cleared by alone. For primes we use the rank bound of Lemma 3.2.
Lemma 3.1.
For an indeterminate ,
| (3.1) |
Proof.
The denominators of a entry divide . An off-diagonal entry is a difference of harmonic sums divided by , so its denominator divides , and so does that of . The entries are integral. The denominators of a entry divide . ∎
3.1. Residue classes
For and let
| (3.2) | ||||
and
| (3.3) |
Lemma 3.2.
Let be a prime with , so that , and let . Then
| (3.4) |
Proof.
Multiply the whole rows of , and by , and . The resulting matrix has and entries in , and because . Write , with . Only the harmonic terms with in the denominator survive modulo . Their quotients are -units because all indices are less than , and , so
The scaled and entries vanish modulo between distinct residue classes modulo . For a cross-class entry, the numerator is -integral after the row scaling and the remaining factor is zero modulo . Within a class, for , with , so the quotient denominator is a unit. Diagonal entries are given by the separate cases in the definitions of and . Since , for
A row reduces to zero unless . Then its entry is , without the factor , and . So, after permutations, the reduction has blocks with rows and columns, where .
For , , the block, with its rows divided by the unit , is
| (3.5) |
of rank one for every . Every other block has rank at most the minimum of its dimensions. So the rank of modulo is at most . The matrix has rows. Row operations over , independent of , make its last rows divisible by coefficientwise. Subtracting the scaling exponent gives the first inequality in (3.4).
Every boundary in (3.2) has the form with . At , , it lies in . The conditions are closed on the left and open on the right in , and the condition is . So is constant on each , and
| (3.6) |
The lines that bound the sets (3.2) are and . The first meets the strip exactly for , the second exactly for . For and for the upper endpoint is . So meets the strip for some if and only if , and if and only if . Hence, for , the sets (3.2) change only on the lines
| (3.7) |
Cut at the finitely many values of where two of these lines meet in , and let be the set of these cut points. For and each exactly one line meets , namely , and exactly one line does, namely . These indices lie in the ranges of (3.7). So on each open interval of the order of the lines in is fixed, exactly five lines lie there, the sets (3.2) are constant on each of the six -intervals between consecutive lines, and is affine. A change of by moves each of these lines by and the endpoint of the integral by . So the sets (3.2) change only on strips of total length . As is bounded, , and is continuous. Table A.2 (Appendix A) lists consecutive intervals with union up to finitely many points, and the affine function on each. Each row follows from (3.2)–(3.3) and (3.7). The affine functions of adjacent rows agree at all internal joins. Summation of the rows gives
| (3.8) |
The configuration , , of (3.5) means , and . For this is , which is nonempty only for . So it occupies for , and for . As , its contribution to (3.8) is
3.2. Larger primes
For define row potentials , , and column potentials by
and put
Lemma 3.3.
For a prime one has .
Proof.
A entry has a pole only at , of order at most one. A entry has pole order at most
including the double pole of for . A entry has a simple pole when . As , there are no higher powers.
In the first range every row has potential at least one, and two if . A collision has , and its column gives the second unit. In the second range all . The columns cover the simple poles and the poles. A collision needs , and its row gives the other unit. In the third range and , so only simple poles remain. In these three ranges every row has potential one, which covers and . In the fourth range , only poles remain, and the rows with are those containing one. So dominates every pole order. Multiply each row and column by to the power of its potential. The potentials add up to , , and in the four ranges. ∎
3.3. The factor at three
Lemma 3.4.
Let . Then
| (3.9) |
In particular .
Proof.
Work in . Write
Subtract times each row from the paired row of . Comparison of coefficients, including the diagonal, gives
| (3.10) | ||||
For example, the off-diagonal lower entry is , and the diagonal entry is . Each coefficient of involves finitely many terms.
Expand along the rows of , and expand each remaining -row minor by the rank-one summands in (3.10). A nonzero term selects distinct rank-one summands. At weight at most
occur: the first summand needs , the third needs . All entries, including , have nonnegative order in . So every term has order at least the sum of the smallest available weights. The infinite expansion is used coefficientwise. Modulo only finitely many occur, so Cauchy–Binet applies to a finite truncation.
3.4. Summation over primes
Put . With the first applicable line taking precedence, let
| (3.11) |
Lemma 3.5.
Proof.
By (3.6), . Lemmas 3.1, 3.2, 3.3 and 3.4 give for every . The exponent at three may be negative. Since and , every inner exponent is at most .
If , then implies . Put
By Fact 1, . For an affine on , Stieltjes integration by parts gives
There are finitely many pieces, and the primes at the finitely many points of , at most one for each point and each , contribute . For the first powers contribute , and the higher powers, including those of two and three,
Subtract once. For the exponent is with
With (3.8) this gives . The total error is . The bounds and give the digits of and of . ∎
4. Analytic estimate
In (2.5) there are , and variables , , . Let
For finite positive measures put
For a fixed let , ,
For a compact line segment or circle let be arclength on . A segment meets the open disk in length at most . A circle meets it in arclength at most . If is at least the radius, this follows from the length of the circle. Otherwise the intersection is an arc of at most a semicircle, of length at most times its chord. So for all , . A positive measure is tame if for some and compact segments or circles , that is, if is a finite sum of measures with arclength density at most on finitely many segments and circles. Tame measures are finite and compactly supported, and sums of tame measures are tame. Let be the set of triples of tame measures of masses with and for some . On these supports .
Lemma 4.1.
For , with formed from ,
| (4.1) |
Proof.
We first show that for tame
By the definition of tame it suffices to take , . Let bound on . For the layer-cake formula and the bound on give
and integration over gives the bound.
Let be the space of real signed measures with tame and . By this bound is finite on and on pairs of tame measures, so all pairings below are finite. For one has , and Fact 2 gives
So is a positive semidefinite symmetric form on , and .
Put , and . The -parts are supported in (the smaller ). The map sends into a line or circle and into , and on . So for a compact segment or circle . Hence and are tame, , and is bounded on . From
and the zero masses,
With , and ,
Let be the symmetric bilinear form with . Then and
Lemma 4.2 (computer-assisted).
Proof.
By Appendix C, , and the interval part of are sums of , and intervals of positive mass , each carrying . The intervals of each part form a chain of adjacent intervals, contained in for , . The circular part is a sum of Poisson measures with and ,
Its arclength density is at most in (4.7). The masses are , and , and is invariant under conjugation. Let be the largest values of over the intervals of the three parts. So the parts of are tame, , and . An interval has potential
with on the real line and . A Poisson measure has potential
| (4.4) |
Energy. Interval pairs use , with . A pair of Poisson measures , gives , by (4.4) and harmonicity. Interval–Poisson pairs integrate (4.4) with the primitives of and , split at . For the denominator, with and ,
Table B.1 gives the outward enclosures, at -bit precision. The enclosure of contains the partial sum minus . The interval sum of the rows gives .
Small arguments. Put . For , , , and . Hence
Both are below the caps.
Subdivision. The bounds on , , and the upper half of are proved on the closed pieces (leaves) of Lemma C.1. The lower half of the circle follows by conjugation. All bounds are computed in Arb ball arithmetic [6] at -bit precision.
Let be a leaf on a real support, with centre and half-width . The interval components of come from the chains of on -leaves and of , on - and -leaves. The endpoints of the chains of , , cut the initial pieces of the -, -, -support. The chain of misses the -support, and that of meets the -support only at . So the open leaf meets at most one interval of each of these chains, and then lies in it. A -leaf lies in or in . Write , where is the sum of the interval components of , with their coefficient , whose interval contains . Inside its interval a component is convex, since there, so lies below its chord on . With , Taylor’s formula with integral remainder for at gives
because the chord plus an affine function is affine. The other interval components have on . The terms and are concave, and on . The terms , are expanded as with the first moments. The omitted value is at most , the omitted derivative lies in , and the second derivative is at most . On the real line a Poisson measure has second derivative for and for ; uses enclosures of these second derivatives over the leaf.
On an arc , , of the circle we take the smaller of two bounds. The first is
with derivatives in and
Here uses enclosures over the arc of these second derivatives for the Poisson terms (). For an interval component of mass and density on it uses the smaller upper end of two enclosures: of the exact second derivative , and of with ranging over . The second bound uses that the distance from to a point increases with . It bounds the real-interval terms by their value at the right end of the arc and applies the first bound to the Poisson terms alone. On the arc starting at only the second bound is used.
For this put
| (4.5) |
Since ,
Lemma 4.3.
Let be a compactly supported positive measure of mass with for all , . If , then
The components of the reference of Lemma 4.2 are compactly supported, and for them one may take
| (4.6) |
the sum over the Poisson measures . It is finite, since for every centre.
Proof.
The closed disk is the intersection of the , , so its -measure is also at most . Let . On both distances are at most , and the layer-cake formula gives
Off , , the logarithmic difference is at most , and
The sum is at most .
By the bounds on before Lemma 4.1, an interval of density at most gives , and a Poisson measure gives : since and arclength is , its arclength density is at most
| (4.7) |
Put , , , and
| (4.8) |
and are finite and depend only on .
Lemma 4.4.
Proof.
Expand in (2.5) into products of its pieces and take absolute values. By (2.4) the paths contribute a factor at most , and has volume one. So it suffices to bound for , , . If there is nothing to prove. Otherwise and the points of each species are distinct. Let , , put mass at each , , . Since , and ,
| (4.10) |
where omits the diagonal in the three self-energies.
Put . Replace every atom by the uniform measure of mass on the circle , and let be the result. Since and the common translation cancels in differences, averaging does not decrease any off-diagonal positive pair. Each circle has energy , and there are atoms. The function is harmonic in each variable on the product of two smearing disks, since . Its mean is , because and . With the masses ,
| (4.11) |
The measures and lie in : is a sum of uniform measures on circles, inside for and , with on the -circles. For this is part of Lemma 4.2. By Lemmas 4.1 and 4.2,
Each point of a smearing circle lies within of the atom it replaces. Since increases on , Lemma 4.3 bounds the change of by . For , and , , so changes by at most . Counting masses, the coefficients of are , and , and the terms give . Hence
The atoms lie in , and . Adding the original external terms of (4.10) turns into , and (4.3) gives
with . Multiply by , put , add the logarithm of the prefactor in (2.5) and . By (4.5) this is (4.9).
For the last statement drop and . As , and . So the right-hand side is at most . ∎
5. Nonvanishing
Let , and let be the -adic valuation, .
Lemma 5.1.
For , coefficientwise in ,
| (5.1) |
Proof.
Lemma 5.2.
Let . For every integer with , .
Proof.
Put , and , and
| (5.3) |
This is the determinant obtained after by multiplying the , and rows by , and . The middle rows become . Every nonzero index, difference and Cauchy denominator has absolute value less than , hence -adic valuation at most . So and lie in , and for
with . So all entries of the scaled matrix lie in . Hence .
Let . Only the summation indices divisible by survive, and , so
The scaled and entries vanish modulo between distinct residue classes modulo . In the class write , . Then , , and . The only row in this class is , with
Cross-class and entries have -adic valuation at least . Within a class the nonzero differences satisfy and are units. All other scaled entries vanish modulo . So, after permutations, the matrix has diagonal blocks
where the subscript refers to . Each block has eight rows with the factor . Hence
| (5.4) |
coefficientwise. In particular .
Take . Then , and since , . By (5.3), . ∎
The restriction is needed in this argument: has the roots and in .
6. Proof of Theorem 1
Appendix A Arithmetic integration
Table A lists the lines (3.7) for the seam . The index ranges are the integer solutions of and (Section 3).
Table A.2 gives, for , the function . Consecutive rows share an endpoint. The first row starts at and the last ends at . The rows are the open intervals of , so a row can be checked at one interior point : the six cells of (3.2) at give an affine function of , which must equal . Then
| family | lines | count |
|---|---|---|
| strip | , | |
| lower | , | |
| lower | , | |
| upper | , | |
| upper | , | |
| , | ||
| total |
Appendix B Certified values
Table B.1 gives the energy of . Here and are the interval and Poisson parts of , is the energy of , , is the energy of , and
Every displayed interval is outward rounded, and the row of contains the tail . The interval sum of the first eight rows, with these coefficients, exceeds , which is (4.2).
| term | outward enclosure |
|---|---|
| [-1.604087582665193, |
|
| [-2.279694146532524, |
|
| [-0.159655877431972, |
|
| [0.226965417324502, |
|
| [-0.495113146744583, |
|
| [0.278231287125003, |
|
| [-0.191796788958401, |
|
| [1.195386024949843, |
|
| [-5.193571445508505, |
Appendix C Reference measure and subdivision
Terminating decimals are exact. Other values are printed as fractions .
C.1. Intervals
Table C.1 gives , and . Each is a chain of adjacent intervals. Its heading gives the start point , and its row is the interval carrying . The three chains have , and rows and end at , and . Outside its chain a part vanishes. The masses are , and .
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C.2. Poisson measures
Table C.2 gives . A row with stands for the two Poisson measures on with centres and , each of mass . The row , with real, stands for one Poisson measure. The measures are those of the proof of Lemma 4.2, and their logarithmic potential is (4.4). This gives measures of total mass .
| 0 | 42 | ||||||
| 1 | 43 | ||||||
| 2 | 44 | ||||||
| 3 | 45 | ||||||
| 4 | 46 | ||||||
| 5 | 47 | ||||||
| 6 | 48 | ||||||
| 7 | 49 | ||||||
| 8 | 50 | ||||||
| 9 | 51 | ||||||
| 10 | 52 | ||||||
| 11 | 53 | ||||||
| 12 | 54 | ||||||
| 13 | 55 | ||||||
| 14 | 56 | ||||||
| 15 | 57 | ||||||
| 16 | 58 | ||||||
| 17 | 59 | ||||||
| 18 | 60 | ||||||
| 19 | 61 | ||||||
| 20 | 62 | ||||||
| 21 | 63 | ||||||
| 22 | 64 | ||||||
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| 24 | 66 | ||||||
| 25 | 67 | ||||||
| 26 | 68 | ||||||
| 27 | 69 | ||||||
| 28 | 70 | ||||||
| 29 | 71 | ||||||
| 30 | 72 | ||||||
| 31 | 73 | ||||||
| 32 | 74 | ||||||
| 33 | 75 | ||||||
| 34 | 76 | ||||||
| 35 | 77 | ||||||
| 36 | 78 | ||||||
| 37 | 79 | ||||||
| 38 | 80 | ||||||
| 39 | 81 | ||||||
| 40 | 82 | ||||||
| 41 | 83 |
C.3. Dyadic subdivision
The four supports are
the last parametrized by . The initial pieces of a support are the closed intervals between consecutive points of the following sets, in increasing order:
-
:
, , and the endpoints of the -chain in ;
-
:
, , , and the endpoints of the -chain in ;
-
:
, , and the endpoints of the -chain in ;
-
:
, .
Since the chains start at , and and end at , and , and is not an endpoint of the -chain, there are , , and initial pieces.
Lemma C.1.
Let , , , . Bisect every initial piece of the support to depth : a piece gives the leaves
in on the real supports and in on the circle. The leaves of each support have rational endpoints, disjoint interiors and union equal to the support. There are , , and leaves, in all (Table C.3).
Proof.
Within a piece the leaves are consecutive and their lengths add up to . The pieces of a support are consecutive and cover it. The counts are products. ∎
The leaves are the closed pieces of the proof of Lemma 4.2. On each of them the bound of that proof is below the cap, so no piece needs a finer subdivision.
| support | initial pieces | depth | leaves |
|---|---|---|---|
| real | |||
| total |
Appendix D Computer checks
The ancillary scripts recheck printed data and run finite tests (Table D). They use exact rational arithmetic and, for the analytic bounds, Arb ball arithmetic [6] through python-flint 0.9.0 under Python 3.12.3.
| check | statements checked |
|---|---|
| determinant | the exact seed determinant: (2.6) and Table 2.1. Finite tests: times the right side of (2.3) with , for and ; the four determinant evaluations in Lemma 2.2 and their product for and three choices of . |
| arithmetic | recomputes the arrangement of (3.7) for in exact rational arithmetic; checks the line families of Table A, every row of Table A.2 (by evaluating from (3.2)–(3.3) at two interior points), (3.8), in (3.12), and ; rational bounds for and ; integrality of for small . |
| analytic | Lemmas 4.2 and C.1: the chains, masses, conjugation symmetry and of Tables C.1 and C.2; Tables B.1, B.2 and C.3, (4.2), (4.3) and , with the bound of the proof of Lemma 4.2 on each of the leaves. |
| mod | (5.2), (5.1), and (5.4) for at . |
Appendix E Proof of Fact 2
For let on , a centred Gaussian density. Then and . For , , and the integrand has constant sign. With and this gives, for ,
The hypothesis of Fact 2 makes the diagonal -null and, by the second identity, integrable for . By Fubini’s theorem and (for each , is bounded, so ),
For fixed , ( the Lebesgue measure on ), and the corresponding triple integral against is at most . So, by Fubini again,
Hence . Compact support is not used.
Use of AI tools
The author devised the method of proof. Codex (OpenAI, GPT-6.1 Sol) computed the reference measure and the other printed data, and wrote the Python scripts in anc/. Claude Code (Anthropic, Claude Opus 5.5) checked the scripts, the data and the tables. The author checked every step of the proof, every table and every reference, and takes full responsibility for the content of the paper.
Statements and declarations
Competing interests. The author has no competing interests.
Funding. No funding was received for this work.
Data availability. All data used in the proof are printed in
Appendices A–C. The anc/
attachment holds the scripts of Appendix D and a
README.md. The entry points are check_
890192149e125730cc0b9b0f85ddd7dab8e9ac391a6d14703849fb74d54f8cd7.
References
- [1] A. Baker, Transcendental Number Theory, 2nd ed., Cambridge Mathematical Library, Cambridge University Press, Cambridge, 1990. MR1074572
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