A068674 Numbers which yield primes when a 3 is prefixed or appended.

1, 7, 11, 17, 31, 37, 59, 67, 73, 109, 119, 121, 137, 169, 187, 191, 221, 229, 259, 271, 331, 343, 359, 361, 373, 407, 413, 449, 467, 499, 511, 527, 533, 539, 541, 557, 581, 607, 613, 617, 637, 673, 701, 719, 733, 739, 767, 779, 793, 823, 851, 889, 917, 929

Offset: 1

Author

Amarnath Murthy, Mar 02 2002

Keywords

• base
• easy
• nonn

Examples

			73 is a term as both 373 and 733 are primes.
		

Crossrefs

Cf. A068673.

Programs

Maple

i := 0:for k from 1 to 3200 do if(isprime(3+10*k) and isprime(k+10^(ceil(evalf(log(k+1)/log(10))))*3)) then s[i] := k; i := i+1; end if; end do:q := seq(s[j],j=0..i-1);

Mathematica

Select[Range[930],Union[PrimeQ[FromDigits/@{Append[(x=IntegerDigits[#]),3],Prepend[x,3]}]]=={True} &] (* Jayanta Basu, May 20 2013 *)

Extensions

More terms from Sascha Kurz, Mar 17 2002