A167768 First of 4 or more consecutive integers with equal values of phi(phi(n)).

1, 7, 31, 32, 2694, 131071, 50802031105

Offset: 1

Author

Fred Schneider, Nov 11 2009

Keywords

• more
• nonn
• hard

Comments

Next term > 1266046940 (last term of A167767 b-file). - Michel Marcus, Jun 23 2013

No other terms < 1.32*10^12. - Jud McCranie, Jul 19 2017

Examples

			p2(1) = p2(2) = p2(3) = p2(4) = 1, p2(7) = p2(8) = p2(9) = p2(10) = 2.
		

Crossrefs

Cf. A167767 (of which this list is a subset), A167766.

Programs

Mathematica

a[n_] := EulerPhi[EulerPhi[n]]; Select[Range[10000], a[#] == a[# + 1] && a[# + 1] == a[# + 2] && a[# + 2] == a[# + 3] &] (* G. C. Greubel, Jun 23 2016 *)

PARI

pp(n) = eulerphi(eulerphi(n))
isA167768(n) = pp(n)==pp(n+1) && pp(n+1)==pp(n+2) && pp(n+2)==pp(n+3) \\ Michael B. Porter, Nov 24 2009

Extensions

Edited by N. J. A. Sloane, Nov 12 2009

a(7) added by Jud McCranie, Jul 17 2017