A367394 Number of integer partitions of n whose length is a semi-sum of the parts.

0, 0, 1, 0, 1, 1, 3, 3, 6, 7, 14, 15, 25, 30, 46, 54, 80, 97, 139, 169, 229, 282, 382, 461, 607, 746, 962, 1173, 1499, 1817, 2302, 2787, 3467, 4201, 5216, 6260, 7702, 9261, 11294, 13524, 16418, 19572, 23658, 28141, 33756, 40081, 47949, 56662, 67493, 79639

Offset

0,7

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.

Links

Table of n, a(n) for n=0..49.

Example

For the partition y = (3,3,2,1) we have 4 = 3 + 1, so y is counted under a(9).
The a(2) = 1 through a(10) = 14 partitions:
  (11)  .  (211)  (221)  (321)   (421)   (521)    (621)    (721)
                         (2211)  (2221)  (2222)   (3222)   (3322)
                         (3111)  (3211)  (3221)   (3321)   (3331)
                                         (3311)   (4221)   (4222)
                                         (32111)  (4311)   (4321)
                                         (41111)  (32211)  (5221)
                                                  (42111)  (5311)
                                                           (32221)
                                                           (33211)
                                                           (42211)
                                                           (43111)
                                                           (331111)
                                                           (421111)
                                                           (511111)

Mathematica

Table[Length[Select[IntegerPartitions[n], MemberQ[Total/@Subsets[#, {2}], Length[#]]&]], {n, 0, 10}]

Crossrefs

The following sequences count and rank integer partitions and finite sets according to whether their length is a subset-sum, linear combination, or semi-sum of the parts. The current sequence is starred.

sum-full sum-free comb-full comb-free semi-full semi-free

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partitions: A367212 A367213 A367218 A367219 A367394* A367398

strict: A367214 A367215 A367220 A367221 A367395 A367399

subsets: A367216 A367217 A367222 A367223 A367396 A367400

ranks: A367224 A367225 A367226 A367227 A367397 A367401

A000041 counts partitions, strict A000009.

A002865 counts partitions whose length is a part, complement A229816.

A236912 counts partitions containing no semi-sum, ranks A364461.

A237113 counts partitions containing a semi-sum, ranks A364462.

A237668 counts sum-full partitions, sum-free A237667.

A366738 counts semi-sums of partitions, strict A366741.

Triangles:

A008284 counts partitions by length, strict A008289.

A365543 counts partitions with a subset-sum k, strict A365661.

A367404 counts partitions with a semi-sum k, strict A367405.

Cf. A000700, A088809, A093971, A126796, A238628, A304792, A363225, A364534, A365541, A365924, A367402.

Sequence in context: A027187 A056508 A050065 * A298732 A078477 A336077

Adjacent sequences: A367391 A367392 A367393 * A367395 A367396 A367397

Keyword

nonn

Author

Gus Wiseman, Nov 19 2023

Status

approved