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

Offset: 1

Author

Frank Ellermann, May 17 2001

Keywords

• nonn

Examples

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

Links

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

Crossrefs

Cf. A005728, A055197, A006842, A006843, A002088, A000010, A055201.

Programs

Mathematica

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

Formula

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

Extensions

More terms from Vladeta Jovovic, Jun 03 2001

Offset corrected by Amiram Eldar, Mar 01 2020