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A087531 Primes consisting only of digits 3 and 7 occurring with equal frequency. 2
37, 73, 33373777, 33773737, 37373773, 37377337, 73337377, 73337773, 73373737, 73773373, 77337373, 77733373, 3337737737, 3337777337, 3377733773, 3733377377, 3733733777, 3737733773, 3773333777, 3773337377, 3773733737, 3777333773 (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

Table of n, a(n) for n=0..21.

MATHEMATICA

Select[Flatten[Table[FromDigits/@Permutations[Join[Table[3, {i}], Table[7, {i}]]], {i, 7}]], PrimeQ] (from Harvey Dale)

PROG

(PARI) /* Primes consisting only of digits x and y, occurring with equal frequency. */ x=3; 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, A087530.

Sequence in context: A083990 A123815 A289732 * A283718 A085959 A145936

Adjacent sequences:  A087528 A087529 A087530 * A087532 A087533 A087534

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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Last modified November 22 03:20 EST 2019. Contains 329383 sequences. (Running on oeis4.)