

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




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


LINKS



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



STATUS

approved



