A072416 a(1) = 2; a(n) = half of the a(n-1)-th even nontotient number.

2, 13, 49, 149, 373, 835, 1727, 3383, 6341, 11419, 19966, 34067, 56967, 93578, 151313, 241281, 379934, 591413, 910879, 1389289, 2099905, 3147743, 4682362, 6914234, 10141072, 14779023, 21408820, 30837491, 44179913, 62974570, 89329823

Offset: 1

Author

Labos Elemer, Jun 17 2002

Keywords

• nonn

Crossrefs

Cf. A006508, A071255, A005277, A072415.

Programs

Mathematica

a = Table[0, {10^6}]; Do[b = EulerPhi[n]/2; If[b < 10^6 + 1, a[[b]] = 1], {n, 1, 2 10^7}]; b = Select[ Range[10^6], a[[ # ]] == 0 &]; c = 2; Do[ Print[c]; c = b[[c]], {n, 0, 17}]

Formula

a(5) = 373 = 746/2, where 746 is the a(4) = 149th term in A005277.

Extensions

Edited by Robert G. Wilson v, Jun 20 2002

a(18)-a(31) from Donovan Johnson, Oct 29 2010