A245359 Largest number k such that d_1^j + d_2^j + … + d_r^j is prime for all j = 1, 2, .. k, or 0 if no such k exists, where d_1, d_2, … d_r are the digits of n. a(n) = -1 if k is infinite.

0, 1, 1, 0, 1, 0, 1, 0, 0, 0, -1, 2, 0, 2, 0, 2, 0, 0, 0, 1, 2, 0, 2, 0, 2, 0, 0, 0, 1, 1, 0, 2, 0, 1, 0, 0, 0, 2, 0, 0, 2, 0, 1, 0, 0, 0, 1, 0, 2, 1, 0, 2, 0, 0, 0, 2, 0, 2, 0, 0, 2, 0, 0, 0, 2, 0, 1, 0, 0, 1, 0, 0, 0, 1, 0, 1, 0, 0, 0, 0, 0, 0, 2, 0, 2, 0, 0, 0, 1, 0, 0, 1, 0, 2

Offset

1,12

Comments

If a(n) = K and reorder the digits of n to make a new number, n'. Thus, a(n') = K.

Links

Table of n, a(n) for n=1..94.

Formula

a(A031974(n)) = -1 for all n.

Example

1^1 + 2^1 = 3 is prime.
1^2 + 2^2 = 5 is prime.
1^3 + 2^3 = 9 is not prime.
So a(12) and a(21) = 2.

Prog

(PARI) a(n) = for(k=1, 10^3, d=digits(n); if(!ispseudoprime(sum(i=1, #d, d[i]^k)), return(k-1))); return(-1)
n=1; while(n<100, print1(a(n), ", "); n++)

Crossrefs

Sequence in context: A373917 A179212 A105118 * A103271 A029832 A361879

Adjacent sequences: A245356 A245357 A245358 * A245360 A245361 A245362

Keyword

sign,base

Author

Derek Orr, Jul 18 2014

Status

approved