A068673 Numbers which yield primes when a 1 is prefixed or appended.

1, 3, 7, 13, 27, 31, 49, 57, 63, 81, 91, 97, 99, 103, 109, 117, 123, 129, 151, 153, 181, 187, 193, 201, 213, 231, 237, 259, 279, 297, 301, 319, 327, 367, 409, 423, 427, 439, 459, 483, 487, 493, 523, 543, 553, 559, 571, 579, 601, 609, 613, 621, 627, 657, 669

Offset: 1

Author

Amarnath Murthy, Mar 02 2002

Keywords

• base
• easy
• nonn

Examples

			57 is a term as 157 as well as 571 are primes.
		

Programs

Maple

i := 0:for k from 1 to 3200 do if(isprime(1+10*k) and isprime(k+10^(ceil(evalf(log(k+1)/log(10))))*1)) then s[i] := k; i := i+1; end if; end do:q := seq(s[j],j=0..i-1);

Mathematica

p1Q[n_]:=Module[{idn=IntegerDigits[n]},PrimeQ[FromDigits[Join[{1}, idn]]] && PrimeQ[FromDigits[Join[idn,{1}]]]]; Select[Range[700],p1Q] (* Harvey P. Dale, Jan 23 2013 *)
Flatten@Position[{#+10^IntegerLength@#,1+10*#}&/@Range@10000,{?PrimeQ..}] (* _Hans Rudolf Widmer, Aug 21 2024 *)

Extensions

More terms from Sascha Kurz, Mar 17 2002