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 A257917 a(n) is the largest y that is a member of a pair (x, y) of integers with x - y > 1 such that x^2 - y^2 is equal to the Fermat number 2^(2^n) + 1, or 0 if no such number exists. 2
 0, 0, 0, 0, 0, 3349888, 33640210518272, 2852314775548000778752, 46730819857678988884581779099803448292025618770199631109363712 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,6 COMMENTS 2^(2^n) + 1 belongs to A019434 if and only if a(n) = 0. REFERENCES M. Krizek, F. Luca, L. Somer, 17 Lectures on Fermat Numbers: From Number Theory to Geometry, CMS Books in Mathematics, vol. 9, Springer-Verlag, New York, 2001, p. 6. LINKS Wikipedia, Fermat number FORMULA If 2^(2^n) + 1 is composite, then a(n) = A257916(n) - A093179(n). PROG (PARI) a(n) = {my(fn = 2^(2^n) + 1); if (isprime(fn), return(0)); my(spf = factor(fn)[1, 1]); (fn/spf - spf)/2; } \\ Michel Marcus, Jun 07 2015 CROSSREFS Cf. A000215, A257916. Sequence in context: A238153 A083635 A116306 * A257916 A015408 A036472 Adjacent sequences:  A257914 A257915 A257916 * A257918 A257919 A257920 KEYWORD nonn AUTHOR Arkadiusz Wesolowski, May 12 2015 STATUS approved

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Last modified January 24 13:24 EST 2020. Contains 331193 sequences. (Running on oeis4.)