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Ramanujan numbers (A000594) read mod 4.
4

%I #15 Nov 27 2017 12:42:19

%S 1,0,0,0,2,0,0,0,1,0,0,0,2,0,0,0,2,0,0,0,0,0,0,0,3,0,0,0,2,0,0,0,0,0,

%T 0,0,2,0,0,0,2,0,0,0,2,0,0,0,1,0,0,0,2,0,0,0,0,0,0,0,2,0,0,0,0,0,0,0,

%U 0,0,0,0,2,0,0,0,0,0,0,0,1,0,0,0,0,0,0,0,2,0,0,0,0,0,0,0,2,0,0,0,2,0,0,0,0

%N Ramanujan numbers (A000594) read mod 4.

%H Antti Karttunen, <a href="/A126812/b126812.txt">Table of n, a(n) for n = 1..65537</a>

%H H. P. F. Swinnerton-Dyer, <a href="http://dx.doi.org/10.1007/978-3-540-37802-0_1">On l-adic representations and congruences for coefficients of modular forms</a>, pp. 1-55 of Modular Functions of One Variable III (Antwerp 1972), Lect. Notes Math., 350, 1973.

%t Mod[#, 4] & /@ RamanujanTau@ Range@ 105 (* _Michael De Vlieger_, Nov 26 2017 *)

%o (PARI) A126812(n) = (ramanujantau(n)%4); \\ _Antti Karttunen_, Nov 26 2017

%Y Cf. A000594, A008442, A098108, A126813, A126814.

%K nonn

%O 1,5

%A _N. J. A. Sloane_, Feb 25 2007