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

1,2

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

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

Sequence in context: A285317 A198592 A325377 * A118618 A163411 A335669

Adjacent sequences: A061557 A061558 A061559 * A061561 A061562 A061563

Keyword

nonn

Author

Frank Ellermann, May 17 2001

Extensions

More terms from Vladeta Jovovic, Jun 03 2001

Offset corrected by Amiram Eldar, Mar 01 2020

Status

approved