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URL: https://oeis.org/A090744

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A090744
Consider numbers of the form ...53197531975319753, whose digits read from the right are 3,5,7,9,1,3,5,7,9,1,3,... Sequence gives lengths of these numbers that are primes.
3
1, 2, 5, 26, 1932, 13422, 16971
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
Sequence in context: A023362 A138613 A299104 * A041571 A042259 A100105
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