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A087537 Primes consisting only of digits 7 and 9 occurring with equal frequency. 2
79, 97, 77997979, 79779979, 79797997, 79997977, 99977797, 99979777, 7777997999, 7779799799, 7779799997, 7779979979, 7797799979, 7797979799, 7799797799, 7977799799, 7979799977, 7979979779, 7979979797, 7979979977, 7979997797 (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..20.

PROG

(PARI) /* Primes consisting only of digits x and y, occurring with equal frequency. */ x=7; y=9; 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, A087536.

Sequence in context: A065211 A106757 A285228 * A117840 A296012 A193142

Adjacent sequences:  A087534 A087535 A087536 * A087538 A087539 A087540

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 September 20 13:45 EDT 2020. Contains 337264 sequences. (Running on oeis4.)