A363887 Number of divisors of 7*n-5 of form 7*k+3.
0, 1, 0, 0, 2, 0, 0, 2, 0, 0, 2, 0, 0, 2, 1, 0, 2, 0, 0, 2, 0, 0, 2, 0, 2, 2, 0, 0, 2, 0, 0, 2, 0, 0, 4, 0, 0, 2, 0, 0, 2, 1, 0, 2, 2, 0, 2, 0, 0, 2, 0, 0, 2, 0, 2, 2, 0, 0, 4, 0, 0, 2, 0, 0, 4, 0, 0, 2, 0, 0, 2, 0, 0, 2, 2, 2, 2, 0, 0, 2, 0, 0, 3, 0, 2, 2, 0, 0, 2, 0, 0, 2, 2, 0, 4, 0, 0, 2, 0, 0
Offset: 1
Keywords
Programs
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Mathematica
a[n_] := DivisorSum[7*n - 5, 1 &, Mod[#, 7] == 3 &]; Array[a, 100] (* Amiram Eldar, Jun 26 2023 *)
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PARI
a(n) = sumdiv(7*n-5, d, d%7==3);
Formula
a(n) = A363805(7*n-5).
G.f.: Sum_{k>0} x^(3*k-1)/(1 - x^(7*k-4)).