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Search: id:A087537
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| A087537 |
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Primes consisting only of digits 7 and 9 occurring with equal frequency. |
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+0
2
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| 79, 97, 77997979, 79779979, 79797997, 79997977, 99977797, 99979777, 7777997999, 7779799799, 7779799997, 7779979979, 7797799979, 7797979799, 7799797799, 7977799799, 7979799977, 7979979779, 7979979797, 7979979977, 7979997797
(list; graph; listen)
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OFFSET
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0,1
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COMMENT
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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).
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MAPLE
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Primes consisting only of digits x and y, occurring with equal frequency. PARI CODE: 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)
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CROSSREFS
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Cf. A087510, A087511, A087536.
Adjacent sequences: A087534 A087535 A087536 this_sequence A087538 A087539 A087540
Sequence in context: A044039 A065211 A106757 this_sequence A117840 A033250 A139922
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KEYWORD
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base,nonn
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AUTHOR
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Amarnath Murthy and Paul D. Hanna (pauldhanna(AT)juno.com) (amarnath_murthy(AT)yahoo.com), Sep 12 2003
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