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 A087515 Primes consisting only of digits 1 and 9 occurring with equal frequency. 6
 19, 11919199, 11919991, 19111999, 19199119, 19911919, 19991911, 91919911, 91999111, 99111919, 99119191, 99919111, 1111919999, 1111999199, 1191911999, 1191919991, 1191991991, 1191999119, 1199911919, 1199911991, 1199919191 (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. MAPLE Primes consisting only of digits x and y, occurring with equal frequency. 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, A087513, A087514. Sequence in context: A172839 A177920 A145308 * A038028 A013890 A347353 Adjacent sequences: A087512 A087513 A087514 * A087516 A087517 A087518 KEYWORD base,nonn AUTHOR Amarnath Murthy and Paul D. Hanna, Sep 11 2003 STATUS approved

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Last modified June 16 15:23 EDT 2024. Contains 373432 sequences. (Running on oeis4.)