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 A068673 Numbers which yield primes when a 1 is prefixed or appended. 18
 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 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 LINKS EXAMPLE 57 is a term as 157 as well as 571 are primes. 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 *) CROSSREFS Sequence in context: A205521 A125898 A146928 * A298360 A140465 A301594 Adjacent sequences:  A068670 A068671 A068672 * A068674 A068675 A068676 KEYWORD base,easy,nonn AUTHOR Amarnath Murthy, Mar 02 2002 EXTENSIONS More terms from Sascha Kurz, Mar 17 2002 STATUS approved

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Last modified January 19 18:13 EST 2020. Contains 331051 sequences. (Running on oeis4.)