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 A087536 Primes consisting only of digits 6 and 7 occurring with equal frequency. 3
 67, 67766767, 76767667, 6666767777, 6667677677, 6667776767, 6667777667, 6676766777, 6676767677, 6676776677, 6677666777, 6677667767, 6677676767, 6766677767, 6766776677, 6767667677, 6767677667, 6776766677, 6776766767 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,1 COMMENTS There are 18 digit pairs which can produce such primes. (1,0),(7,0),(1,3),(1,4),(1,6),(1,7),(1,9),(2,3),(2,9),(3,4),(3,5),(3,7),(3,8),(4,7),(4,9),(5,9),(6,7),(7,9). LINKS Harvey P. Dale, Table of n, a(n) for n = 0..5000 MATHEMATICA Sort[Flatten[Table[Select[FromDigits/@Permutations[PadRight[{}, 2n, {6, 7}]], PrimeQ], {n, 5}]]] (* Harvey P. Dale, Dec 03 2012 *) PROG (PARI) /* Primes consisting only of digits x and y, occurring with equal frequency. */ x=6; y=7; d1=x; d2=y; k=0; a=vector(100); for(n=1, 3000, B=binary(n); L=length(B); s=sum(j=1, length(B), B[j]); if(L%2==0 & s==L/2, C=vector(L, n, (d2-d1)*B[n]+d1); p=subst(Pol(C), x, 10); if(isprime(p), if(k<100, k++; a[k]=p)); D=vector(L, n, d2-(d2-d1)*B[n]); q=subst(Pol(D), x, 10); if(isprime(q ), if(k<100, k++; a[k]=q))); ); a=vector(k, n, a[n]); vecsort(a) CROSSREFS Cf. A087510, A087511, A087535. Sequence in context: A263463 A279798 A191941 * A281888 A282338 A198210 Adjacent sequences:  A087533 A087534 A087535 * A087537 A087538 A087539 KEYWORD base,nonn AUTHOR Amarnath Murthy and Paul D. Hanna, Sep 12 2003 EXTENSIONS Edited by Charles R Greathouse IV, Oct 28 2009 STATUS approved

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