0,1

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 Sierpiński'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

Conjecture: 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

It appears that this sequence is generated by starting with a(0)=3 and following the rule "Write in binary and read in base 4".  For an example of "Write in binary and read in ternary", see A014118. - John W. Layman, Jul 30 2013

Conjecture: the numbers > 5 in this sequence, i.e., 2^2^k + 1 for k>1, are exactly the numbers n such that (n-1)^4-1 divides 2^(n-1)-1. - M. F. Hasler, Jul 24 2015

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.

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.

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.

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.

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 de theoremate quodam Fermatiano aliisque ad numeros primos spectantibus

Emmanuel Ferrand, Deformations of the Taylor Formula, Journal of Integer Sequences, Vol. 10 (2007), Article 07.1.7.

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

T.-W. Leung, A Brief Introduction to Fermat Numbers

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, Fermat Numbers

R. Munafo, Notes on Fermat numbers

S. Mustonen, On integer sequences with mutual k-residues

OEIS Wiki, Fermat numbers

OEIS Wiki, Sierpinski's triangle

G. A. Paxson, The compositeness of the thirteenth Fermat number, Math. Comp. 15 (76) (1961) 420-420.

C. Pomerance, A tale of two sieves, Notices Amer. Math. Soc., 43 (1996), 1473-1485.

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

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 [edited by Daniel Forgues, Jun 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

If a(n) is composite, then a(n) = A242619(n)^2 + A242620(n)^2 = A257916(n)^2 - A257917(n)^2. - Arkadiusz Wesolowski, May 13 2015

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).

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, Mar 21 2009]

Table[2^(2^n) + 1, {n, 0, 8}] (* Alonso del Arte, Jun 07 2011 *)

(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 */

(Haskell)

a000215 = (+ 1) . (2 ^) . (2 ^)  -- Reinhard Zumkeller, Feb 13 2015

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: A254576 A232720 A247203 * A123599 A100270 A016045

Adjacent sequences:  A000212 A000213 A000214 * A000216 A000217 A000218

nonn,easy,nice

N. J. A. Sloane

approved