A240679 - OEIS

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A240679

Primes p such that p*10+k is prime for exactly two values of the digit k.

2, 3, 5, 17, 23, 37, 59, 67, 73, 97, 101, 127, 149, 157, 193, 197, 211, 223, 229, 233, 239, 241, 269, 283, 331, 337, 349, 353, 373, 379, 401, 433, 439, 463, 467, 479, 487, 499, 571, 607, 613, 619, 631, 673, 691, 701, 733, 757, 769, 811, 853, 859, 937, 941

(list; graph; refs; listen; history; text; internal format)

OFFSET

1,1

LINKS

Colin Barker, Table of n, a(n) for n = 1..1000

EXAMPLE

2 is in the sequence because 23 and 29 are prime, but 21 and 27 are not prime.

MATHEMATICA

Select[Prime[Range[200]], Count[Table[10 #+k, {k, {1, 3, 7, 9}}], _?PrimeQ] == 2&] (* Harvey P. Dale, Jan 24 2019 *)

PROG

(PARI) forprime(p=2, 1500, t=0; forstep(k=1, 9, 2, if(isprime(p*10+k), t++)); if(t==2, print1(p, ", ")))

CROSSREFS