OFFSET
2,4
COMMENTS
We define a semi-sum of a multiset to be any sum of a 2-element submultiset. This is different from sums of pairs of elements. For example, 2 is the sum of a pair of elements of {1}, but there are no semi-sums.
EXAMPLE
The partition y = (3,2,1,1) has semi-sum 3 = 2+1, but no semi-sum 6, so y is counted under T(7,3) but not under T(7,6).
Triangle begins:
1
1 1
2 1 2
3 2 2 2
5 3 4 2 3
7 5 6 4 3 3
11 7 9 6 6 3 4
15 11 13 10 9 6 4 4
22 15 20 13 15 9 8 4 5
30 22 27 21 21 15 12 8 5 5
42 30 39 28 30 21 20 12 10 5 6
56 42 53 41 42 33 28 20 15 10 6 6
77 56 73 55 60 42 44 28 25 15 12 6 7
Row n = 7 counts the following partitions:
(511) (421) (331) (421) (511) (61)
(4111) (3211) (322) (4111) (421) (52)
(3211) (2221) (3211) (322) (331) (43)
(31111) (22111) (31111) (3211)
(22111) (211111) (2221)
(211111) (22111)
(1111111)
MATHEMATICA
Table[Length[Select[IntegerPartitions[n], MemberQ[Total/@Subsets[#, {2}], k]&]], {n, 2, 10}, {k, 2, n}]
CROSSREFS
KEYWORD
nonn,tabl
AUTHOR
Gus Wiseman, Nov 17 2023
STATUS
approved