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Apr 8, 2013 at 0:16 comment added ya-tayr Since the formal series $f(x) = \sum_k \sum_A A^k x^k = \sum_A (I - Ax)^{-1}$ doesn't change if you replace the summand by $(I - (A+I)x)^{-1}$, the series satisfies $f(x) = (1-x) f(x/(1-x))$, which is a little bit like being a modular form of weight 1.
Apr 7, 2013 at 22:14 comment added paul garrett Wonderfully clear!
Apr 7, 2013 at 20:21 history answered Sergei Ivanov CC BY-SA 3.0