A372639 - OEIS

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A372639

a(n) = (1/4) * Sum_{k=1..n} phi(10*k).

1, 3, 5, 9, 14, 18, 24, 32, 38, 48, 58, 66, 78, 90, 100, 116, 132, 144, 162, 182, 194, 214, 236, 252, 277, 301, 319, 343, 371, 391, 421, 453, 473, 505, 535, 559, 595, 631, 655, 695, 735, 759, 801, 841, 871, 915, 961, 993, 1035, 1085, 1117, 1165, 1217, 1253, 1303

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

OFFSET

1,2

LINKS

Table of n, a(n) for n=1..55.

Eric Weisstein's World of Mathematics, Totient Function.

FORMULA

a(n) ~ (25/(6*Pi^2)) * n^2. - Amiram Eldar, May 08 2024

MATHEMATICA

Accumulate[Table[EulerPhi[10*n], {n, 1, 60}]]/4 (* Amiram Eldar, May 08 2024 *)

PROG

(PARI) a(n) = sum(k=1, n, eulerphi(10*k))/4;