A281947 - OEIS

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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

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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).

LINKS

Table of n, a(n) for n=1..28.

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

Cf. A000010, A002202, A181062, A281909.

Sequence in context: A083809 A092967 A056431 * A199466 A199966 A011027

Adjacent sequences: A281944 A281945 A281946 * A281948 A281949 A281950

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