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Write numbers from 1 to n(n+1)/2 in a triangular array (a) downwards and (b) across; a(n) is number of numbers in same position in both.

+0
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3, 4, 4, 5, 4, 4, 6, 4, 6, 4, 6, 6, 4, 6, 4, 6, 6, 4, 6, 4, 5, 8, 4, 10, 4, 4, 6, 6, 10, 4, 4, 6, 4, 6, 4, 4, 10, 6, 6, 4, 6, 10, 4, 6, 6, 6, 8, 4, 6, 8, 4, 6, 6, 8, 6, 6, 6, 6, 8, 6, 4, 10, 6, 6, 6, 4, 10, 6, 6, 4, 4, 8, 4, 6, 6, 6, 6, 6, 14, 4, 6, 6, 4, 6, 4, 4, 10, 4, 10, 6, 4, 10, 6, 10, 6, 10, 10, 4 (list; graph; listen)

	OFFSET	`0,1`
	FORMULA	`For n>1, a(n) = 2 + d(2n^2 - 6n + 5), where d(k) is number of divisors of k (Dean Hickerson, dean.hickerson(AT)yahoo.com).`
	EXAMPLE	`For n=3, (a) = 1; 2 4; 3 5 6, (b) = 1; 2 3; 4 5 6, so a(3)=4.`
	CROSSREFS	`Sequence in context: A112180 A158012 A032446 this_sequence A107574 A053405 A082090` `Adjacent sequences: A028946 A028947 A028948 this_sequence A028950 A028951 A028952`
	KEYWORD	`nonn,easy`
	AUTHOR	`Jim Propp (propp(AT)math.wisc.edu)`
	EXTENSIONS	`More terms from David G Radcliffe (radcl008(AT)umn.edu)`

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Last modified November 25 20:09 EST 2009. Contains 167514 sequences.

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