A072342 - OEIS

A072342

a(n) = the least natural number k such that k*reverse(n) + 1 is prime.

1, 1, 2, 1, 2, 1, 4, 2, 2, 1, 2, 2, 10, 2, 2, 6, 8, 2, 6, 1, 1, 1, 3, 1, 1, 5, 1, 1, 3, 2, 4, 2, 2, 4, 2, 2, 4, 2, 4, 1, 2, 3, 3, 2, 2, 3, 2, 4, 3, 2, 2, 4, 2, 4, 6, 2, 2, 12, 2, 1, 1, 2, 1, 1, 2, 1, 3, 2, 1, 4, 6, 4, 4, 6, 4, 4, 6, 4, 4, 2, 1, 1, 5, 2, 1, 2, 1, 1, 2, 2, 10, 2, 2, 4, 12, 2, 4, 2, 2, 1, 6, 8

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OFFSET

1,3

COMMENTS

Conjecture: a(n) is less than or equal to n for all n.

LINKS

Table of n, a(n) for n=1..102.

EXAMPLE

Reverse(12) = 21 and the least natural number k such that 21 k + 1 is prime is k = 2; so a(12) = 2.

MATHEMATICA

f[n_] := Module[{i}, i = 0; While[ ! PrimeQ[i*FromDigits[Reverse[IntegerDigits[n]]]+ 1], i++ ]; i]; Table[f[i], {i, 1, 150}]

CROSSREFS