A372622 - OEIS

A372622

a(n) = (1/4) * Sum_{k=1..n} phi(5*k).

1, 2, 4, 6, 11, 13, 19, 23, 29, 34, 44, 48, 60, 66, 76, 84, 100, 106, 124, 134, 146, 156, 178, 186, 211, 223, 241, 253, 281, 291, 321, 337, 357, 373, 403, 415, 451, 469, 493, 513, 553, 565, 607, 627, 657, 679, 725, 741, 783, 808, 840, 864, 916, 934, 984, 1008, 1044

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OFFSET

1,2

LINKS

Table of n, a(n) for n=1..57.

Eric Weisstein's World of Mathematics, Totient Function.

FORMULA

a(n) ~ (25/(8*Pi^2)) * n^2. - Amiram Eldar, May 08 2024

MATHEMATICA

Accumulate[Table[EulerPhi[5*n], {n, 1, 60}]]/4 (* Amiram Eldar, May 08 2024 *)

PROG

(PARI) a(n) = sum(k=1, n, eulerphi(5*k))/4;