A281947 Smallest prime p such that p^i - 1 is a totient (A002202) for all i = 1 to n, or 0 if no such p exists.

2, 3, 7, 7, 37, 37, 113, 113, 241, 241, 241, 241, 241, 241, 241, 241, 241, 241, 2113, 2113, 2113, 2113, 2113, 2113, 3121, 3121, 3121, 3121

Offset: 1

Author

Altug Alkan, Feb 03 2017

Keywords

• nonn
• more

Comments

p - 1 = phi(p) is a totient for all primes p.

If A281909(n) is prime, then a(n) = A281909(n).

Examples

			a(3) = 7 because 7^2 - 1 = 48, 7^3 - 1 = 342 are both totient numbers (A002202) and 7 is the least prime number with this property.
		

Crossrefs

Cf. A000010, A002202, A181062, A281909.

Programs

PARI

isok(p, n)=for (i=1, n, if (! istotient(p^i-1), return(0));); 1;
a(n) = {my(p=2); while (! isok(p, n), p = nextprime(p+1)); p;} \\ Michel Marcus, Feb 04 2017

Extensions

a(19) from Michel Marcus, Feb 04 2017

a(20)-a(28) from Ray Chandler, Feb 08 2017