A372638 - OEIS

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A372638

a(n) = (1/6) * Sum_{k=1..n} phi(7*k).

1, 2, 4, 6, 10, 12, 19, 23, 29, 33, 43, 47, 59, 66, 74, 82, 98, 104, 122, 130, 144, 154, 176, 184, 204, 216, 234, 248, 276, 284, 314, 330, 350, 366, 394, 406, 442, 460, 484, 500, 540, 554, 596, 616, 640, 662, 708, 724, 773, 793, 825, 849, 901, 919, 959, 987, 1023

(list; graph; refs; listen; history; text; internal format)

OFFSET

1,2

LINKS

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

Eric Weisstein's World of Mathematics, Totient Function.

FORMULA

a(n) ~ (49/(16*Pi^2)) * n^2. - Amiram Eldar, May 08 2024

MATHEMATICA

Accumulate[Table[EulerPhi[7*n], {n, 1, 60}]]/6 (* Amiram Eldar, May 08 2024 *)

PROG

(PARI) a(n) = sum(k=1, n, eulerphi(7*k))/6;