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A000215
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Fermat numbers: 2^(2^n) + 1, n >= 0.
(Formerly M2503 N0990)
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135
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3, 5, 17, 257, 65537, 4294967297, 18446744073709551617, 340282366920938463463374607431768211457, 115792089237316195423570985008687907853269984665640564039457584007913129639937
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OFFSET
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0,1
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COMMENTS
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It is conjectured that just the first 5 numbers in this sequence are primes.
An infinite coprime sequence defined by recursion. - Michael Somos Mar 14 2004
For n>0, Fermat numbers F(n) have digital roots 5 or 8 depending on whether n is even or odd (Koshy). - Lekraj Beedassy, Mar 17 2005
This is the special case k=2 of sequences with exact mutual k-residues. In general, a(1)=k+1 and a(n)=min{m | m>a(n-1), mod(m,a(i))=k, i=1,...,n-1}. k=1 gives Sylvester's sequence A000058. - Seppo Mustonen, Sep 04 2005
For n>1 final two digits of a(n) are periodically repeated with period 4: {17, 57, 37, 97}. - Alexander Adamchuk, Apr 07 2007
For 1<k<=2^n, a(A007814(k-1)) divides a(n)+2^k. More generally, for any number k, let r=mod(k,2^n) and suppose r != 1, then a(A007814(r-1)) divides a(n)+2^k. - T. D. Noe, Jul 12 2007
A000120(a(n)) = 2. [Reinhard Zumkeller, Aug 07 2010]
From Daniel Forgues, Jun 20 2011: (Start)
The Fermat numbers F_n are F_n(a,b) = a^(2^n) + b^(2^n) with a = 2 and b = 1.
All factors of F_n = 2^(2^n)+1 are of the form k*(2^n)+1, k >= 1.
The products of distinct Fermat numbers (in their binary representation, see A080176) give rows of Sierpinski's triangle (A006943). (End)
Let F(n) be a Fermat number. For n > 2, F(n) is prime if and only if 5^((F(n)-1)/4) == sqrt(F(n)-1) (mod F(n)). [Arkadiusz Wesolowski, Jul 16 2011]
Let the smallest prime factor of Fermat number F(n) be P(F(n)). If F(n) is composite, then P(F(n)) < 3*2^(2^n/2 - n - 2). - Arkadiusz Wesolowski, Aug 10 2012
The Fermat primes are not Brazilian numbers, so they belong to A220627, but the Fermat composites are Brazilian numbers so they belong to A220571. For a proof, see Proposition 3 page 36 on "Les nombres brésiliens" in Links. [Bernard Schott, Dec 29 2012]
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REFERENCES
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M. Aigner and G. M. Ziegler, Proofs from The Book, Springer-Verlag, Berlin, 2nd. ed., 2001; see p. 3.
T. M. Apostol, Introduction to Analytic Number Theory, Springer-Verlag, 1976, page 7.
P. Bachmann, Niedere Zahlentheorie (1902, 1910), reprinted Chelsea, NY, 1968, vol. 2, p. 87.
Emmanuel Ferrand, Deformations of the Taylor Formula, Journal of Integer Sequences, Vol. 10 (2007), Article 07.1.7.
James Gleick, Faster, Vintage Books, NY, 2000 (see pp. 259-261).
R. K. Guy, Unsolved Problems in Number Theory, A3.
G. H. Hardy and E. M. Wright, An Introduction to the Theory of Numbers. 3rd ed., Oxford Univ. Press, 1954, p. 14.
E. Hille, Analytic Function Theory, Vol. I, Chelsea, N.Y., see p. 64.
T. Koshy, "The Digital Root Of A Fermat Number", Journal of Recreational Mathematics Vol. 32 No. 2 2002-3 Baywood NY.
M. Krizek, F. Luca & L. Somer, 17 Lectures on Fermat Numbers, Springer-Verlag NY 2001.
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
C. S. Ogilvy and J. T. Anderson, Excursions in Number Theory, Oxford University Press, NY, 1966. pp. 36.
Clifford A. Pickover, A Passion for Mathematics, Wiley, 2005; see p. 18, 59.
C. A. Pickover, The Math Book, Sterling, NY, 2009; see p. 202.
C. Pomerance, A tale of two sieves, Notices Amer. Math. Soc., 43 (1996), 1473-1485.
N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
D. Wells, The Penguin Dictionary of Curious and Interesting Numbers, pp. 148-9 Penguin Books 1987.
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LINKS
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N. J. A. Sloane, Table of n, a(n) for n = 0..11
C. K. Caldwell, The Prime Glossary, Fermat number
L. Euler, Observations on a theorem of Fermat and others on looking at prime numbers, arXiv:math.HO/0501118
L. Euler, Observationes do theoremate quodam Fermatiano aliisque ad numeros primos spectantibus
Wilfrid Keller, Prime factors k.2^n + 1 of Fermat numbers F_m
T.-W. Leung, A Brief Introduction to Fermat Numbers
R. Munafo, Fermat Numbers
R. Munafo, Notes on Fermat numbers
S. Mustonen, On integer sequences with mutual k-residues
G. A. Paxson, The compositeness of the thirteenth Fermat number, Math. Comp. 15 (76) (1961) 420-420.
P. Sanchez, PlanetMath.org, Fermat Numbers
Bernard Schott, Les nombres brésiliens, Quadrature, no. 76, avril-juin 2010, pages 30-38. Local copy, included here with permission from the editors of Quadrature.
G. Villemin's Almanach of Numbers, Nombres de Fermat
Eric Weisstein's World of Mathematics, Fermat Number
Eric Weisstein's World of Mathematics, Generalized Fermat Number
Wikipedia, Fermat number
Wolfram Research, Fermat numbers are pairwise coprime
OEIS Wiki, Fermat numbers
OEIS Wiki, Sierpinski's triangle
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FORMULA
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a(0) = 3; a(n) = (a(n-1)-1)^2 + 1, n >= 1.
a(n) = a(n-1)*a(n-2)*...*a(1)*a(0) + 2, n >= 0, where for n = 0, we get the empty product, i.e. 1, plus 2, giving 3 = a(0). - Benoit Cloitre, Sep 15 2002 [edit by Daniel Forgues, June 20 2011]
The above formula implies that the Fermat numbers (being all odd) are coprime.
Conjecture: F is a Fermat prime if and only if phi(F-2) = (F-1)/2. - Benoit Cloitre, Sep 15 2002
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EXAMPLE
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a(0) = 1*2^1+1 = 3 = 1*(2*1)+1;
a(1) = 1*2^2+1 = 5 = 1*(2*2)+1;
a(2) = 1*2^4+1 = 17 = 2*(2*4)+1;
a(3) = 1*2^8+1 = 257 = 16*(2*8)+1;
a(4) = 1*2^16+1 = 65537 = 2048*(2*16)+1;
a(5) = 1*2^32+1 = 4294967297 = 641*6700417 = (10*(2*32)+1)*(104694*(2*32)+1);
a(6) = 1*2^64+1 = 18446744073709551617 = 274177*67280421310721
= (2142*(2*64)+1)*(525628291490*(2*64)+1);
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MAPLE
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A000215 := n->2^(2^n)+1;
with(numtheory):a[1]:=0: for n from 0 to 26 do a[n]:=fermat(n) od: seq(a[n], n=0..9); # [From Zerinvary Lajos (zerinvarylajos(AT)yahoo.com), Mar 21 2009]
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MATHEMATICA
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Table[2^(2^n) + 1, {n, 0, 8}] (* From Alonso del Arte, Jun 07 2011 *)
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PROG
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(PARI) a(n)=if(n<1, 3*(n==0), (a(n-1)-1)^2+1)
(Maxima) A000215(n):=2^(2^n)+1$ makelist(A000215(n), n, 0, 10); /* Martin Ettl, Dec 10 2012 */
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CROSSREFS
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a(n) = A001146(n) + 1 = A051179(n) + 2.
Cf. A019434, A050922, A051179, A063486, A073617, A085866.
See A004249 for a similar sequence.
Cf. A080176 for binary representation of Fermat numbers.
Cf. A220627, A220570, A220571, A125134.
Sequence in context: A067387 A050922 A070592 * A123599 A100270 A016045
Adjacent sequences: A000212 A000213 A000214 * A000216 A000217 A000218
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KEYWORD
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nonn,easy,nice
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AUTHOR
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N. J. A. Sloane.
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STATUS
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approved
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