A105581 Primes whose indices are palindromic.

2, 3, 5, 7, 11, 13, 17, 19, 23, 31, 79, 137, 193, 257, 317, 389, 457, 523, 547, 607, 661, 739, 811, 877, 947, 1019, 1087, 1153, 1231, 1301, 1399, 1459, 1531, 1601, 1667, 1747, 1831, 1907, 1999, 2081, 2141, 2239, 2309, 2381, 2447, 2549, 2647, 2699, 2777, 2851

Offset: 1

Author

Cino Hilliard, May 03 2005

Keywords

• easy
• nonn
• base

Examples

			prime(22) = 79 prime(33) = 137 the 11th and 12th entries.
		

Crossrefs

Cf. A258433.

Programs

Mathematica

Prime[#]&/@Select[Range[500],PalindromeQ] (* Harvey P. Dale, Nov 24 2022 *)

PARI

g(n) = for(x=1,n,if(ispal(x),print1(prime(x)","))) ispal(n) = { local(len,s1,s2); n=Str(n); len=length(n)\2; s1 = left(n,len); s2 = right(n,len); if(s1 == rev(s2),return(1),return(0)) } left(str,n) = \ Get the left n characters from string str. { local(v,tmp,x); v =""; tmp = Vec(str); ln=length(tmp); if(n > ln,n=ln); for(x=1,n, v=concat(v,tmp[x]); ); return(v) } right(str,n) = \ Get the right n characters from string str. { local(v,ln,s,x); v =""; tmp = Vec(str); ln=length(tmp); if(n > ln,n=ln); s = ln-n+1; for(x=s,ln, v=concat(v,tmp[x]); ); return(v) } rev(str) = \ Get the reverse of the input string { local(tmp,s,j); tmp = Vec(Str(str)); s=""; forstep(j=length(tmp),1,-1, s=concat(s,tmp[j])); return(s) }