login
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.
0
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
1,1
COMMENTS
p - 1 = phi(p) is a totient for all primes p.
If A281909(n) is prime, then a(n) = A281909(n).
EXAMPLE
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.
PROG
(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
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Altug Alkan, Feb 03 2017
EXTENSIONS
a(19) from Michel Marcus, Feb 04 2017
a(20)-a(28) from Ray Chandler, Feb 08 2017
STATUS
approved