OFFSET
1,2
COMMENTS
a(5) - Fermat and Lucas PRP! Tested with pfgw64 with switch -tc. - Marek Hubal, Mar 04 2019
a(6) - Fermat and Lucas PRP! Tested with pfgw64 with switch -tc. a(7) > 32000. - Marek Hubal, Mar 06 2019
EXAMPLE
a(1)=1 because 3 is prime and 3 has 1 digit.
a(2)=2 because 53 is prime and 53 has 2 digits.
a(3)=5 because 19753 is prime and 19753 has 5 digits.
MATHEMATICA
s = 0; Do[s = s + 10^n*Switch[ Mod[n, 5], 4, 1, 0, 3, 1, 5, 2, 7, 3, 9]; If[ PrimeQ[s], Print[n + 1]], {n, 0, 3000}] (* Robert G. Wilson v, Feb 19 2004 *)
PROG
(PARI) test3(n)= s=0; for(i=0, n, if(Mod(i, 5)==0, s=s+3*10^i, if(Mod(i, 5)==1, s=s+5*10^i, if(Mod(i, 5)==2, s=s+7*10^i, if(Mod(i, 5)==3, s=s+9*10^i, if(Mod(i, 5)==4, s=s+1*10^i, )))))); return(s);
for(j=0, 1000, if(isprime(test3(j)), print1(j+1, ", ")));
CROSSREFS
KEYWORD
hard,nonn,base,more
AUTHOR
Mohammed Bouayoun (bouyao(AT)wanadoo.fr), Feb 03 2004
EXTENSIONS
a(4) from Robert G. Wilson v, Feb 10 2004
a(5) from Marek Hubal, Mar 04 2019
a(6) from Marek Hubal, Mar 06 2019
STATUS
approved