A363850 - OEIS

%I #14 Jun 25 2023 10:39:22

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

%T 2,1,3,1,2,1,2,1,1,1,3,2,1,1,2,1,1,1,3,2,1,1,4,1,1,1,3,1,1,1,2,1,1,2,

%U 4,1,1,1,2,1,2,1,3,1,3,1,2,1,1,2,3,1,1,1,2,2,1,1,4,1,1,1,2,1,2,1

%N Number of divisors of 7*n-3 of form 7*k+1.

%C Also number of divisors of 7*n-3 of form 7*k+4.

%F a(n) = A279061(7*n-3) = A363806(7*n-3).

%F G.f.: Sum_{k>0} x^(4*k-3)/(1 - x^(7*k-6)).

%F G.f.: Sum_{k>0} x^k/(1 - x^(7*k-3)).

%t a[n_] := DivisorSum[7*n - 3, 1 &, Mod[#, 7] == 1 &]; Array[a, 100] (* _Amiram Eldar_, Jun 25 2023 *)

%o (PARI) a(n) = sumdiv(7*n-3, d, d%7==1);

%Y Cf. A279061, A363806.

%K nonn

%O 1,5

%A _Seiichi Manyama_, Jun 24 2023