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A061560 Lengths of Farey-series (A005728) which are not primes. 1
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

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

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Last modified July 9 03:39 EDT 2020. Contains 335538 sequences. (Running on oeis4.)