A143978 - OEIS

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A143978

Floor((2n(n+1)/3)).

1, 4, 8, 13, 20, 28, 37, 48, 60, 73, 88, 104, 121, 140, 160, 181, 204, 228, 253, 280, 308, 337, 368, 400, 433, 468, 504, 541, 580, 620, 661, 704, 748, 793, 840, 888, 937, 988, 1040, 1093, 1148, 1204, 1261, 1320, 1380, 1441, 1504, 1568, 1633, 1700, 1768, 1837 (list; graph; refs; listen; history; internal format)

	OFFSET	`1,2`
	COMMENTS	`Second diagonal of array A143979, which counts certain unit squares` `in a lattice. First diagonal: A030511.`
	FORMULA	`Floor(2n(n+1)/3).` `G.f.: x(1+x)^2/((1+x+x^2)(1-x)^3). a(n)= 2a(n-1) -a(n-2) +a(n-3) -2a(n-4) +a(n-5). [From R. J. Mathar (mathar(AT)strw.leidenuniv.nl), Oct 05 2009]`
	CROSSREFS	`Cf. A143979.` `Sequence in context: A064609 A056738 A170907 * A071994 A023661 A157130` `Adjacent sequences: A143975 A143976 A143977 * A143979 A143980 A143981`
	KEYWORD	`nonn`
	AUTHOR	`Clark Kimberling (ck6(AT)evansville.edu), Sep 06 2008`

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Last modified February 16 16:51 EST 2012. Contains 205938 sequences.