A101782 Upper bound of twin prime pairs whose digital reverse is prime.

5, 7, 13, 31, 73, 151, 181, 199, 313, 1021, 1033, 1153, 1231, 1279, 1321, 1429, 1453, 1669, 1723, 1789, 1879, 1933, 3121, 3169, 3301, 3373, 3391, 3463, 3469, 3541, 3583, 3853, 7459, 7561, 7879, 7951, 9001, 9013, 9241, 9421, 9439, 9679, 9721, 9769, 9931

Offset

1,1

Links

Robert Israel, Table of n, a(n) for n = 1..10000

Example

13 is the upper bound of twin prime pair (11,13) and its digital reverse, 31, is prime.

Maple

digrev:= proc(n) local L, i;
  L:= convert(n, base, 10);
  add(L[-i]*10^(i-1), i=1..nops(L))
end proc:
select(t -> isprime(t) and isprime(t-2) and isprime(digrev(t)), [5, seq(i, i=7 .. 10^5, 6)]); # Robert Israel, Dec 08 2024

Prog

(PARI) twurpr(n) = { for(x=1, n, y=twinu(x); z=eval(rev(y)); if(isprime(z), print1(y", ")) ); }
twinu(n) = /* The n-th upper twin prime */ { local(c, x); c=0; x=1; while(c<n, if(isprime(prime(x)+2), c++); x++; ); return(prime(x)); }
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); }

Crossrefs

Sequence in context: A293059 A179625 A141191 * A288889 A168609 A266951

Adjacent sequences: A101779 A101780 A101781 * A101783 A101784 A101785

Keyword

easy,nonn,base

Author

Cino Hilliard, Jan 26 2005

Status

approved