A101781 - OEIS

A101781

Lower bound b of twin primes pairs such that b's digital reverse is also prime.

3, 5, 11, 17, 71, 101, 107, 149, 179, 191, 311, 347, 1031, 1061, 1091, 1151, 1229, 1301, 1487, 1619, 1949, 3251, 3257, 3299, 3359, 3371, 3389, 3467, 3527, 3767, 3821, 3851, 3917, 3929, 7349, 7457, 7547, 7589, 7757, 7949, 9011, 9041, 9341, 9437, 9857, 10007

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OFFSET

1,1

LINKS

Harvey P. Dale, Table of n, a(n) for n = 1..1000

EXAMPLE

17 is the lower bound of twin prime pair (17,19) and its digital reverse, 71, is prime.

MATHEMATICA

Select[Transpose[Select[Partition[Prime[Range[1500]], 2, 1], Last[#]- First[#] == 2&]][[1]], PrimeQ[FromDigits[Reverse[IntegerDigits[#]]]]&] (* Harvey P. Dale, Jun 26 2013 *)

Select[Partition[Prime[Range[1500]], 2, 1], #[[2]]-#[[1]]==2&&PrimeQ[IntegerReverse[#[[1]]]]&][[;; , 1]] (* Harvey P. Dale, Sep 01 2025 *)

PROG

(PARI) twinl(n) = /* The n-th lower twin prime */ { local(c, x); c=0; x=1; while(c<n, if(isprime(prime(x)+2), c++); x++; ); return(prime(x-1)) }

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) }

twlrpr(n) = { for(x=1, n, y=twinl(x); z=eval(rev(y)); if(isprime(z), print1(y", ")) ) }

CROSSREFS

Sequence in context: A284306 A283399 A216181 * A101783 A078883 A355901

Adjacent sequences: A101778 A101779 A101780 * A101782 A101783 A101784

KEYWORD

nonn,base

AUTHOR

Cino Hilliard, Jan 26 2005

EXTENSIONS

Definition clarified by Harvey P. Dale, Sep 01 2025

STATUS

approved