Ramanujan's partition generating functions modulo $\ell$
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| Format: | Preprint |
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2025
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| _version_ | 1866911194465435648 |
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| author | Bringmann, Kathrin Craig, William Ono, Ken |
| author_facet | Bringmann, Kathrin Craig, William Ono, Ken |
| contents | For the partition function $p(n)$, Ramanujan proved the striking identities $$
P_5(q):=\sum_{n\geq 0} p(5n+4)q^n =5\prod_{n\geq 1} \frac{\left(q^5;q^5\right)_{\infty}^5}{(q;q)_{\infty}^6}, $$ $$
P_7(q):=\sum_{n\geq 0} p(7n+5)q^n =7\prod_{n\geq 1}\frac{\left(q^7;q^7\right)_{\infty}^3}{(q;q)_{\infty}^4}+49q
\prod_{n\geq 1}\frac{\left(q^7;q^7\right)_{\infty}^7}{(q;q)_{\infty}^8}, $$ where $(q;q)_{\infty}:=\prod_{n\geq 1}(1-q^n).$ As these identities imply his celebrated congruences modulo 5 and 7, it is natural to seek, for primes $\ell \geq 5,$ closed form expressions of the power series
$$
P_{\ell}(q):=\sum_{n\geq 0} p(\ell n-δ_{\ell})q^n\pmod{\ell},
$$
where $δ_{\ell}:=\frac{\ell^2-1}{24}.$ In this paper, we prove that
$$
P_{\ell}(q)\equiv c_{\ell} \frac{T_{\ell}(q)}{ (q^\ell; q^\ell )_\infty} \pmod{\ell},
$$
where $c_{\ell}\in \mathbb{Z}$ is explicit and $T_{\ell}(q)$ is the generating function for the Hecke traces of $\ell$-ramified values of special Dirichlet series for weight $\ell-1$ cusp forms on $SL_2(\mathbb{Z})$. This is a new proof of Ramanujan's congruences modulo 5, 7, and 11, as there are no nontrivial cusp forms of weight 4, 6, and 10. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2506_06101 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | Ramanujan's partition generating functions modulo $\ell$ Bringmann, Kathrin Craig, William Ono, Ken Number Theory Combinatorics 11P82, 05A17 For the partition function $p(n)$, Ramanujan proved the striking identities $$ P_5(q):=\sum_{n\geq 0} p(5n+4)q^n =5\prod_{n\geq 1} \frac{\left(q^5;q^5\right)_{\infty}^5}{(q;q)_{\infty}^6}, $$ $$ P_7(q):=\sum_{n\geq 0} p(7n+5)q^n =7\prod_{n\geq 1}\frac{\left(q^7;q^7\right)_{\infty}^3}{(q;q)_{\infty}^4}+49q \prod_{n\geq 1}\frac{\left(q^7;q^7\right)_{\infty}^7}{(q;q)_{\infty}^8}, $$ where $(q;q)_{\infty}:=\prod_{n\geq 1}(1-q^n).$ As these identities imply his celebrated congruences modulo 5 and 7, it is natural to seek, for primes $\ell \geq 5,$ closed form expressions of the power series $$ P_{\ell}(q):=\sum_{n\geq 0} p(\ell n-δ_{\ell})q^n\pmod{\ell}, $$ where $δ_{\ell}:=\frac{\ell^2-1}{24}.$ In this paper, we prove that $$ P_{\ell}(q)\equiv c_{\ell} \frac{T_{\ell}(q)}{ (q^\ell; q^\ell )_\infty} \pmod{\ell}, $$ where $c_{\ell}\in \mathbb{Z}$ is explicit and $T_{\ell}(q)$ is the generating function for the Hecke traces of $\ell$-ramified values of special Dirichlet series for weight $\ell-1$ cusp forms on $SL_2(\mathbb{Z})$. This is a new proof of Ramanujan's congruences modulo 5, 7, and 11, as there are no nontrivial cusp forms of weight 4, 6, and 10. |
| title | Ramanujan's partition generating functions modulo $\ell$ |
| topic | Number Theory Combinatorics 11P82, 05A17 |
| url | https://arxiv.org/abs/2506.06101 |