OFFSET
1,1
MATHEMATICA
Select[Range@ 218, PrimeQ@ First@ IntegerDigits@ # &] (* Michael De Vlieger, Nov 05 2018 *)
PROG
(PARI) isok(n) = isprime(digits(n)[1]); \\ Michel Marcus, Oct 17 2018
(PARI) a(n) = n--; my (w=1); while (n >= 4*w, n -= 4*w; w*=10); w*prime(1+n\w) + (n%w) \\ Rémy Sigrist, Nov 16 2018
CROSSREFS
KEYWORD
nonn,base
AUTHOR
Kritsada Moomuang, Oct 16 2018
STATUS
approved