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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

Table of n, a(n) for n=1..55.

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.)