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%I #15 May 28 2023 12:52:31
%S 2,8,11,21,54,734,5636,14954
%N 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.
%e a(1) = 2 because 19 is prime and 19 has 2 digits.
%e a(2) = 8 because 31975319 is prime and 31975319 has 8 digits.
%t 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 *)
%o (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);
%o for(j=0,732,if(isprime(test9(j)),print1(j+1, ", ")))
%Y Cf. A090743, A090744, A090745.
%K hard,nonn,base,more,less
%O 1,1
%A mohammed bouayoun (bouyao(AT)wanadoo.fr), Feb 03 2004
%E a(6) from _Robert G. Wilson v_, Feb 10 2004
%E a(7) from _Donovan Johnson_, Mar 06 2008
%E Offset changed to 1 and a(8) from _Michael S. Branicky_, May 27 2023
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