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 A215938 Numbers n such that the Fibonacci number F(n) can be written in the form a^2 + 5*b^2. 1
 1, 2, 5, 8, 11, 12, 25, 29, 32, 41, 48, 55, 89, 121, 125, 128, 131, 145, 179, 192, 205, 275, 331, 359, 401, 421, 431, 445, 449, 509, 512, 569, 571, 601, 605, 625, 631, 655, 659, 691, 725, 768, 895, 911, 1025, 1375 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS A number n can be written in the form a^2+5*b^2 if and only if n is 0, or of the form 2^(2i) 5^j Prod_{p==1 or 9 mod 20} p^k Prod_{q==3 or 7 mod 20) q^(2m) or of the form 2^(2i+1) 5^j Prod_{p==1 or 9 mod 20} p^k Prod_{q==3 or 7 mod 20) q^(2m+1), for integers i,j,k,m, for primes p,q. LINKS Blair Kelly, Fibonacci and Lucas factorizations PROG (PARI) for(i=2, 500, a=factorint(fibonacci(i))~; flag=0; flip=0; for(j=1, #a, if(((a[1, j]%20>10))&&a[2, j]%2==1, flag=1); if(((a[1, j]%20==2)||(a[1, j]%20==3)||(a[1, j]%20==7))&&a[2, j]%2==1, flip=flip+1)); if(flag==0&&flip%2==0, print(i", "))) CROSSREFS Cf. A000045, A215939, A124132. Cf. A020669, A033205 (numbers and primes of the form x^2 + 5*y^2). Sequence in context: A187581 A167409 A082406 * A352748 A007826 A108589 Adjacent sequences: A215935 A215936 A215937 * A215939 A215940 A215941 KEYWORD nonn,more AUTHOR V. Raman, Aug 27 2012 EXTENSIONS Terms corrected by V. Raman, Sep 20 2012 a(46) from Amiram Eldar, Oct 14 2019 STATUS approved

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Last modified February 7 09:08 EST 2023. Contains 360115 sequences. (Running on oeis4.)