A331097 - OEIS

A331097

a(n) is the greatest prime number of the form n mod (10^k) for some k > 0, or 0 if no such prime number exists.

0, 0, 2, 3, 0, 5, 0, 7, 0, 0, 0, 11, 2, 13, 0, 5, 0, 17, 0, 19, 0, 0, 2, 23, 0, 5, 0, 7, 0, 29, 0, 31, 2, 3, 0, 5, 0, 37, 0, 0, 0, 41, 2, 43, 0, 5, 0, 47, 0, 0, 0, 0, 2, 53, 0, 5, 0, 7, 0, 59, 0, 61, 2, 3, 0, 5, 0, 67, 0, 0, 0, 71, 2, 73, 0, 5, 0, 7, 0, 79, 0

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OFFSET

0,3

COMMENTS

In other words, a(n) is the largest prime suffix of n, or 0 if no such suffix exists.

LINKS

Rémy Sigrist, Table of n, a(n) for n = 0..10000

FORMULA

a(n) <= n with equality iff n = 0 or n is a prime number.

EXAMPLE

For n = 42:

- 42 mod (10^k) = 42 is not prime for k >= 2,