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A046711
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From the Bruck-Ryser theorem: n == 1 or 2 (mod 4) which are also the sum of 2 squares.
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0
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1, 2, 5, 9, 10, 13, 17, 18, 25, 26, 29, 34, 37, 41, 45, 49, 50, 53, 58, 61, 65, 73, 74, 81, 82, 85, 89, 90, 97, 98, 101, 106, 109, 113, 117, 121, 122, 125, 130, 137, 145, 146, 149, 153, 157, 162, 169, 170, 173, 178, 181, 185, 193, 194, 197, 202, 205, 218, 221, 225
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,2
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COMMENTS
| If n = 1, 2 (mod 4) and the squarefree part of n is divisible by a prime p = 3 (mod 4), then no difference set of order n exists. Equivalently, if a projective plane of order n exists, and n= 1 or 2 (mod 4), then n is the sum of two squares. [Jonathan Vos Post, Apr 17, 2011]
Intersection of A001481 and A042963; A000161(a(n)) > 0. [Reinhard Zumkeller, Feb 14 2012]
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REFERENCES
| R. H. Bruck and H. J. Ryser, The nonexistence of certain projective planes, Canad. J. Math., 1 (1949), 88-93.
M. Hall, Jr., Combinatorial Theory, Theorem 12.3.2.
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LINKS
| Reinhard Zumkeller, Table of n, a(n) for n = 1..10000
Index entries for sequences related to sums of squares
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PROG
| (Haskell)
a046711 n = a046711_list !! (n-1)
a046711_list = [x | x <- a042963_list, a000161 x > 0]
-- Reinhard Zumkeller, Aug 16 2011
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CROSSREFS
| Sequence in context: A078360 A114995 A047619 * A191171 A191776 A095347
Adjacent sequences: A046708 A046709 A046710 * A046712 A046713 A046714
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KEYWORD
| nonn,easy,nice,changed
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AUTHOR
| N. J. A. Sloane (njas(AT)research.att.com).
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EXTENSIONS
| More terms from James A. Sellers (sellersj(AT)math.psu.edu)
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