

A045544


Odd values of n for which a regular ngon can be constructed by compass and straightedge.


18



3, 5, 15, 17, 51, 85, 255, 257, 771, 1285, 3855, 4369, 13107, 21845, 65535, 65537, 196611, 327685, 983055, 1114129, 3342387, 5570645, 16711935, 16843009, 50529027, 84215045, 252645135, 286331153, 858993459, 1431655765, 4294967295
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OFFSET

1,1


COMMENTS

If there are no more Fermat primes, then 4294967295 is the last term in the sequence.
From Daniel Forgues, Jun 17 2011: (Start)
The 31 = 2^5  1 terms of this sequence are the nonempty products of distinct Fermat primes. The 5 known Fermat primes are in A019434.
Prepending the empty product, i.e., 1, to this sequence gives A004729.
The initial term for this sequence is thus a(1) (offset=1), since a(0) should correspond to the omitted empty product, term a(0) of A004729.
Rows 1 to 31 of Sierpiński's triangle, if interpreted as a binary number converted to decimal (A001317), give a(1) to a(31). (End)


LINKS

Table of n, a(n) for n=1..31.
Wilfrid Keller, Prime factors k.2^n + 1 of Fermat numbers F_m
OEIS Wiki, Constructible oddsided polygons
OEIS Wiki, Sierpinski's triangle


FORMULA

Each term is the product of distinct odd Fermat primes.


MATHEMATICA

Union[Times@@@Rest[Subsets[{3, 5, 17, 257, 65537}]]] (* Harvey P. Dale, Sep 20 2011 *)


CROSSREFS

Cf. A019434. Essentially same as A004729.
Coincides with A001317 for the first 31 terms only.  Robert G. Wilson v, Dec 22 2001
Cf. A004729.
Sequence in context: A094358 A003527 A004729 * A001317 A053576 A197818
Adjacent sequences: A045541 A045542 A045543 * A045545 A045546 A045547


KEYWORD

hard,nonn,nice


AUTHOR

Ken Takusagawa


STATUS

approved



