A061560 - OEIS

A061560

Lengths of Farey-series (A005728) which are not primes.

1, 33, 65, 81, 121, 129, 141, 201, 213, 231, 243, 279, 309, 325, 345, 361, 385, 451, 475, 531, 543, 585, 605, 629, 651, 697, 713, 755, 775, 807, 831, 901, 965, 1001, 1029, 1261, 1309, 1329, 1395, 1495, 1565, 1589, 1661, 1737, 1773, 1833, 1857, 1935, 1967

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

OFFSET

1,2

LINKS

Amiram Eldar, Table of n, a(n) for n = 1..10000

FORMULA

a(n) = A005728(A055197(n-1)) for n > 1, a(1) = A005728(0).

EXAMPLE

a(3) = 65 = 5*13 is not a prime, A055197(3-1) = 14, A005728(14) = 65.

MATHEMATICA

seq = {1}; sum = 1; Do[sum += EulerPhi[k]; If[!PrimeQ[sum], AppendTo[seq, sum]], {k, 1, 80}]; seq (* Amiram Eldar, Mar 01 2020 *)

CROSSREFS