Hi, for the bibliography of a paper I'm writing I seek a citation for the first statement of the conjecture that the nontrivial zeros of $F(s) = \sum_n\tau(n)n^{-s}$ all lie on the line Re(s) = 6. (Here tau is the Ramanujan tau function.)

Thanks.

Barry Brent

**Edit:**
I feel (and people have said in print) that Ramanujan must have made the conjecture himself, but I haven't been able to locate the statement. As far as I can see it isn't in the paper (#18 of the Collected Papers, "On certain arithmetical functions") in which he defines the series and conjectures the Euler product.