A240234 Least number k such that k*n^k + 1 is prime, or 0 if no such number exists.

1, 1, 2, 1, 1242, 1, 34, 5, 2, 1, 10, 1

Offset

1,3

Comments

a(n) = 1 if and only if n + 1 is prime.

Next term a(13), if it exists, is greater than 1000000. Other unknown terms are at index: 25, 29, 41, 47, 49, 53, 55, 69, 73, 79. (See links). - Jeppe Stig Nielsen, Aug 08 2014

Links

Table of n, a(n) for n=1..12.

Guenter Loeh, Generalized Cullen primes n b^n + 1

Jeppe Stig Nielsen, Table of a(n) for n = 1..100, with unknown entries denoted by question marks

Example

1*3^1 + 1 = 4 is not prime. 2*3^2 + 1 = 19 is prime. Thus, a(3) = 2.

Prog

(PARI) a(n) = for(k=1, 4000, if(ispseudoprime(k*n^k+1), return(k)))
n=1; while(n<100, print1(a(n), ", "); n++)

Crossrefs

Cf. A089001, A116954.

Sequence in context: A087037 A354913 A036109 * A367996 A098940 A098939

Adjacent sequences: A240231 A240232 A240233 * A240235 A240236 A240237

Keyword

nonn,hard,more

Author

Derek Orr, Apr 02 2014

Status

approved