A220693 - OEIS

A220693

Irregular triangle where the k-th item in the n-th row (both starting from 1) tells in how many ways we can add 2 distinct integers from 1 to n in such a way that the sum is divisible by k. Row n has 2n-1 terms.

0, 1, 0, 1, 3, 1, 1, 1, 1, 6, 2, 2, 1, 2, 1, 1, 10, 4, 4, 2, 2, 2, 2, 1, 1, 15, 6, 5, 3, 3, 2, 3, 2, 2, 1, 1, 21, 9, 7, 5, 4, 3, 3, 3, 3, 2, 2, 1, 1, 28, 12, 10, 6, 6, 4, 4, 3, 4, 3, 3, 2, 2, 1, 1, 36, 16, 12, 8, 8, 5, 5, 4, 4, 4, 4, 3, 3, 2, 2, 1, 1

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OFFSET

1,5

COMMENTS

After the first two rows, this irregular table gives all the nonzero terms from the beginning of each row of A220691. See the comments there and at A061857.

LINKS

Antti Karttunen, Rows n = 1..100 of table, flattened

Index entries for sequences related to subset sums modulo m

FORMULA

See Robert Israel's formula at A061857.

EXAMPLE

Row n (starting from row 1) has 2n-1 terms in this irregular table:

1, 0, 1;

3, 1, 1, 1, 1;

6, 2, 2, 1, 2, 1, 1;

10, 4, 4, 2, 2, 2, 2, 1, 1;

15, 6, 5, 3, 3, 2, 3, 2, 2, 1, 1;