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 A045544 Odd values of n for which a regular n-gon can be constructed by compass and straightedge. 21
 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 (list; graph; refs; listen; history; text; internal format)
 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 Wilfrid Keller, Prime factors k.2^n + 1 of Fermat numbers F_m. OEIS Wiki, Constructible odd-sided polygons. OEIS Wiki, Sierpinski's triangle. FORMULA Each term is the product of distinct odd Fermat primes. Sum_{n>=1} 1/a(n) = -1 + Product_{n>=1} {1+1/A019434(n)) = 0.7007354948... >= 1003212011/1431655765 = sigma(2^32-1)/(2^32-1) - 1, with equality if there are only five Fermat primes (A019434). - Amiram Eldar, Jan 22 2022 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 STATUS approved

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Last modified June 29 17:37 EDT 2022. Contains 354913 sequences. (Running on oeis4.)