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 A086785 Primes found among the numerators of the continued fraction rational approximations to Pi. 2
 3, 103993, 833719, 4272943, 411557987, 7809723338470423412693394150101387872685594299 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS The numbers listed are primes. For m <= 10000 the only occurrence where both numerator and denominator are prime is 833719/265381. The next term has 123 digits. - Harvey P. Dale, Dec 23 2018 LINKS Joerg Arndt, Table of n, a(n) for n = 1..15 Cino Hilliard, Continued fractions rational approximation of numeric constants. [needs login] EXAMPLE The first 4 rational approximations to Pi are 3/1, 22/7, 333/106, 355/113, 103993/33102 where 3 and 103993 are primes. MATHEMATICA Select[Numerator[Convergents[Pi, 100]], PrimeQ] (* Harvey P. Dale, Dec 23 2018 *) PROG (PARI) \\ Continued fraction rational approximation of numeric functions cfrac(m, f) = x=f; for(n=0, m, i=floor(x); x=1/(x-i); print1(i, ", ")) cfracnumprime(m, f) = { cf = vector(100000); x=f; for(n=0, m, i=floor(x); x=1/(x-i); cf[n+1] = i; ); for(m1=0, m, r=cf[m1+1]; forstep(n=m1, 1, -1, r = 1/r; r+=cf[n]; ); numer=numerator(r); denom=denominator(r); if(isprime(numer), print1(numer, ", ")); ) } (PARI) default(realprecision, 10^5); cf=contfrac(Pi); n=0; { for(k=1, #cf,  \\ generate b-file     pq = contfracpnqn( vector(k, j, cf[j]) );     p = pq[1, 1];  q = pq[2, 1];     if ( ispseudoprime(p), n+=1; print(n, " ", p) );  \\ A086785 \\    if ( ispseudoprime(q), n+=1; print(n, " ", q) );  \\ A086788 ); } /* Joerg Arndt, Apr 21 2013 */ CROSSREFS Cf. A002485, A224936. Sequence in context: A164841 A171366 A292691 * A159577 A116536 A224241 Adjacent sequences:  A086782 A086783 A086784 * A086786 A086787 A086788 KEYWORD easy,nonn AUTHOR Cino Hilliard, Aug 04 2003 EXTENSIONS Corrected by Jens Kruse Andersen, Apr 20 2013 Corrected offset, Joerg Arndt, Apr 21 2013 STATUS approved

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Last modified October 23 02:03 EDT 2019. Contains 328335 sequences. (Running on oeis4.)