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| | [No title] (Site not responding. Last check: 2007-11-05) |
 | | In this paper, using the theory of modular forms, we prove seven identities of the following type: $$\sum_{k=0}^{[n/m]}\sigma_r(k)\sigma_s(n-mk)=P\sigma_{r+s+1}(n)+ Qn\sigma_{r+s-1}(n),\tag1 $$ which hold for every $n$ satisfying suitable congruences, for suitable integers $m\ge2$ and $r,s=1$ or 3, and for rationals $P$ and $Q$ (Theorem 2). |
 | | 1,$ the Eisenstein series $$E_{2k}(\tau):=1+\frac2{\zeta(1-2k)}\sum_{n=1}^{\infty}\sigma_{2k-1}(n)q^n$$ are modular forms of weight $2k$ for $\Gamma.$ The function $E_2(\tau):=1-24\sum_{n=1}^\infty\sigma_1(n)q^n$ is not a modular form, but is transformed under the action of $ SL(2,\Bbb Z)$ as follows: $$E_2\left(\frac{a\tau+b}{c\tau+d}\right)=(c\tau+d)^{2}E_2(\tau)+\frac6 {\pi i}c(c\tau+d).$$ We shall also denote $E_{2k,m}(\tau)=E_{2k}(m\tau).$ For $k>1,$ the functions $E_{2k,m}$ are modular forms of weight $2k$ for $\Gamma_0(m).$ \subheading{3. |
 | | With the same notation as above, for a modular form $f$ of weight $2k$ for $\Gamma_0(m)$ we have $$f\left(\frac{a}{c}\right)=\lim_{\tau\rightarrow i\infty} (c\tau+d)^{-2k}f\left(\frac{a\tau+b}{c\tau+d}\right)= \lim_{\varepsilon\rightarrow 0} (c\varepsilon)^{2k} f\left(\frac{a}{c}+\varepsilon\right). |
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