A372621 - OEIS

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A372621

a(n) = (1/2) * Sum_{k=1..n} phi(3*k).

1, 2, 5, 7, 11, 14, 20, 24, 33, 37, 47, 53, 65, 71, 83, 91, 107, 116, 134, 142, 160, 170, 192, 204, 224, 236, 263, 275, 303, 315, 345, 361, 391, 407, 431, 449, 485, 503, 539, 555, 595, 613, 655, 675, 711, 733, 779, 803, 845, 865, 913, 937, 989, 1016, 1056, 1080, 1134

(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) ~ (27/(8*Pi^2)) * n^2. - Amiram Eldar, May 08 2024

MATHEMATICA

Accumulate[Table[EulerPhi[3*n], {n, 1, 60}]]/2 (* Amiram Eldar, May 08 2024 *)

PROG

(PARI) a(n) = sum(k=1, n, eulerphi(3*k))/2;