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A028949 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
1, 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; refs; listen; history; text; internal format)
OFFSET

1,2

LINKS

Table of n, a(n) for n=1..99.

FORMULA

For n>1, a(n) = 2 + d(2n^2 - 6n + 5), where d(k) is number of divisors of k. - Dean Hickerson

EXAMPLE

For n=1, (a) = 1, (b) = 1, so a(1)=1.

For n=3, (a) = 1; 2 4; 3 5 6, (b) = 1; 2 3; 4 5 6, so a(3)=4.

PROG

(PARI) a(n) = if (n==1, 1, 2 + numdiv(2*n^2 - 6*n + 5)) \\ Michel Marcus, Jun 15 2013

CROSSREFS

Sequence in context: A158012 A032446 A271563 * A201006 A107574 A241163

Adjacent sequences:  A028946 A028947 A028948 * A028950 A028951 A028952

KEYWORD

nonn,easy

AUTHOR

James Propp, Dec 11 1999

EXTENSIONS

More terms from David Radcliffe

STATUS

approved

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Last modified April 20 11:10 EDT 2019. Contains 322309 sequences. (Running on oeis4.)