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A050922 Triangle in which n-th row gives prime factors of n-th Fermat number 2^(2^n)+1. 10
3, 5, 17, 257, 65537, 641, 6700417, 274177, 67280421310721, 59649589127497217, 5704689200685129054721, 1238926361552897, 93461639715357977769163558199606896584051237541638188580280321 (list; graph; refs; listen; history; text; internal format)



Alternatively, list of prime factors of terms of A001317 in order of their first appearance. - Labos Elemer, Jan 21 2002

From T. D. Noe, Jan 29 2009: (Start)

That these two definitions give the same sequence follows from the fact (stated as a formula in A001317) that A001317(n) is the product of Fermat numbers F(i) according to which bits of n are set.

For instance, for n=41, the binary representation of n is 101001, which has bits 0, 3 and 5 set. A001317(n) = 3311419785987 = 3*257*4294967297 = F(0)*F(3)*F(5).

This factorization also explains why the "first 31 numbers give odd-sided constructible polygons". I think Hewgill first noticed this factorization. (End)


M. Aigner and G. M. Ziegler, Proofs from The Book, Springer-Verlag, Berlin, 2nd. ed., 2001; see p. 3.


Jeppe Stig Nielsen, Table of n, a(n) for n = 0..29

J. Bernheiden, Fermat Numbers (Text in German)

R. P. Brent, Factorization of the tenth Fermat number

R. P. Brent, Factorization of the eleventh Fermat number

R. P. Brent, Succint proofs of primality for the factors of some Fermat numbers

R. P. Brent & J. M. Pollard, Factorization of the eighth Fermat number

R. P. Brent et al., Three new factors of Fermat numbers

C. K. Caldwell, The Prime Glossary, Fermat divisor

Wilfrid Keller, Prime factors k.2^n + 1 of Fermat numbers F_m

R. Mestrovic, Euclid's theorem on the infinitude of primes: a historical survey of its proofs (300 BC--2012) and another new proof, arXiv preprint arXiv:1202.3670, 2012. - From N. J. A. Sloane, Jun 13 2012

R. Munafo, Notes on Fermat numbers

Mercedes Orús-Lacort, Fermat numbers are not prime numbers for n >= 5, (2020).

Eric Weisstein's World of Mathematics, Fermat Number


Triangle begins:






641, 6700417;

274177, 67280421310721;

59649589127497217, 5704689200685129054721;

1238926361552897, 93461639715357977769163558199606896584051237541638188580280321; ...

A001317(127) =, A001317(128) = 59649589127497217.5704689200685129054721. See also A050922. Compare with A053576, where 2 and A000215 appear as prime factors. - Labos Elemer, Jan 21 2002


Flatten[Transpose[FactorInteger[#]][[1]]&/@Table[2^(2^n)+1, {n, 0, 8}]] (* Harvey P. Dale, May 18 2012 *)


(PARI) for(n=0, 1e3, f=factor(2^(2^n)+1)[, 1]; for(i=1, #f, print1(f[i], ", "))) \\ Felix Fröhlich, Aug 16 2014


Cf. A000215, A019434, A023394, A093179.

Cf. A001317, A001316, A003401, A045544, A053576.

Sequence in context: A125045 A093179 A067387 * A260476 A070592 A254576

Adjacent sequences:  A050919 A050920 A050921 * A050923 A050924 A050925




N. J. A. Sloane, Dec 30 1999


More terms from Larry Reeves (larryr(AT)acm.org), Apr 13 2000.

Edited by N. J. A. Sloane, Jan 31 2009 at the suggestion of T. D. Noe

Link to Munafo webpage fixed by Robert Munafo, Dec 09 2009



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Last modified January 18 08:32 EST 2021. Contains 340250 sequences. (Running on oeis4.)