

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
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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



