A098045 a(n) is the substring of the digits of A082521(n) that precedes the 2n+1 central zeros.

1, 13, 14, 19, 13, 14, 16, 71, 34, 35, 74, 94, 13, 13, 127, 94, 79, 106, 76, 74, 140, 14, 172, 118, 32, 73, 134, 71, 173, 395, 71, 14, 185, 115, 182, 97, 118, 13, 137, 104, 310, 31, 923, 701, 163, 152, 79, 115, 94, 17, 118, 73, 94, 349, 170, 17, 328, 158, 92, 944

Offset: 0

Author

David Wasserman, Sep 10 2004

Keywords

• base
• easy
• nonn
• less

Comments

I interpret A082521 to mean that A082521(n) has at least 2n+1 central zeros. If we require exactly 2n+1 central zeros, then a(21) is 329 instead of 140, a(41) is 328 instead of 310 and a(55) is 182 instead of 170.

Links

• Jeppe Stig Nielsen, Table of n, a(n) for n = 0..800

Programs

PARI

a(n)=forstep(j=10^(n+1),+oo,10^(n+1),d=digits(j);p=fromdigits(vector(2*#d-1,k,if(k<#d,d[k],d[2*#d-k])));ispseudoprime(p)&&return(j/10^(n+1))) \\ Jeppe Stig Nielsen, Apr 07 2025