A218343 a(n) is the largest n-digit palindromic integer surrounded by twin primes, if one exists, or 0 otherwise.

6, 0, 858, 2112, 89898, 276672, 8916198, 89522598, 898373898, 8998558998, 89984248998, 899931139998, 8999867689998, 89999400499998, 899998808899998, 8999950220599998, 89999961316999998, 899999985589999998, 8999999741479999998, 89999993611639999998

Offset: 1

Author

Igor Schein, Oct 26 2012

Keywords

• nonn
• base

Links

• The prime puzzles, Puzzle 659

Programs

PARI

a(n)=my(p,t,v,k,j,d,N=n-1);if(n==1,6,if(n==2,0,t=9*10^(N\2);d=6*11^(N%2);forstep(k=t,0,-1,v=digits(k);v=concat(v,vecextract(Vecrev(v),concat(Str((N+1)%2+1),"..")));p=subst(Pol(v),x,10);if(p%d==0&&ispseudoprime(p+1)&&ispseudoprime(p-1),return(p)))))