A103965 - OEIS

A103965

Numbers k such that the string k10k is the decimal expansion of a prime number.

9, 27, 41, 81, 111, 113, 117, 183, 191, 219, 231, 261, 279, 281, 309, 321, 339, 363, 377, 399, 401, 411, 429, 449, 461, 467, 477, 479, 497, 503, 513, 539, 549, 567, 603, 633, 653, 659, 689, 693, 707, 723, 731, 737, 747, 773, 843, 849, 887, 891, 911, 917

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OFFSET

1,1

LINKS

Table of n, a(n) for n=1..52.

EXAMPLE

k=9 is in the sequence because k10k = 9109 is prime.

k=113 is in the sequence because k10k = 11310113 is prime.

k=219 is in the sequence because k10k = 21910219 is prime.

MAPLE

q:= n-> isprime(parse(cat(n, 10, n))):

select(q, [$1..917])[]; # Alois P. Heinz, Sep 07 2025