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
KEYWORD
nonn,base
AUTHOR
Colin Barker, Apr 10 2014
STATUS
approved