A090746 Consider numbers of the form ...19753197531975319, whose digits read from the right are 9,1,3,5,7,9,1,3,5,7,9,1,... Sequence gives lengths of these numbers that are primes.

2, 8, 11, 21, 54, 734, 5636, 14954

Offset

1,1

Links

Table of n, a(n) for n=1..8.

Example

a(1) = 2 because 19 is prime and 19 has 2 digits.
a(2) = 8 because 31975319 is prime and 31975319 has 8 digits.

Mathematica

s = 0; Do[s = s + 10^n*Switch[ Mod[n, 5], 1, 1, 2, 3, 3, 5, 4, 7, 0, 9]; If[ PrimeQ[s], Print[n + 1]], {n, 0, 3000}] (* Robert G. Wilson v, Feb 10 2004 *)

Prog

(PARI) test9(n)= s=0; for(i=0, n, if(Mod(i, 5)==0, s=s+9*10^i, if(Mod(i, 5)==1, s=s+1*10^i, if(Mod(i, 5)==2, s=s+3*10^i, if(Mod(i, 5)==3, s=s+5*10^i, if(Mod(i, 5)==4, s=s+7*10^i, )))))); return(s);
for(j=0, 732, if(isprime(test9(j)), print1(j+1, ", ")))

Crossrefs

Cf. A090743, A090744, A090745.

Sequence in context: A295070 A009420 A189328 * A362869 A234924 A336771

Adjacent sequences: A090743 A090744 A090745 * A090747 A090748 A090749

Keyword

hard,nonn,base,more,less

Author

mohammed bouayoun (bouyao(AT)wanadoo.fr), Feb 03 2004

Extensions

a(6) from Robert G. Wilson v, Feb 10 2004

a(7) from Donovan Johnson, Mar 06 2008

Offset changed to 1 and a(8) from Michael S. Branicky, May 27 2023

Status

approved