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A321773 Number of compositions of n into parts with distinct multiplicities and with exactly three parts. 5
1, 3, 6, 4, 9, 9, 10, 12, 15, 13, 18, 18, 19, 21, 24, 22, 27, 27, 28, 30, 33, 31, 36, 36, 37, 39, 42, 40, 45, 45, 46, 48, 51, 49, 54, 54, 55, 57, 60, 58, 63, 63, 64, 66, 69, 67, 72, 72, 73, 75, 78, 76, 81, 81, 82, 84, 87, 85, 90, 90, 91, 93, 96, 94, 99, 99 (list; graph; refs; listen; history; text; internal format)
OFFSET

3,2

LINKS

Alois P. Heinz, Table of n, a(n) for n = 3..1000

FORMULA

Conjectures from Colin Barker, Dec 11 2018: (Start)

G.f.: x^3*(1 + 3*x + 5*x^2) / ((1 - x)^2*(1 + x)*(1 + x + x^2)).

a(n) = a(n-2) + a(n-3) - a(n-5) for n>7.

(End)

EXAMPLE

From Gus Wiseman, Nov 11 2020: (Start)

Also the number of 3-part non-strict compositions of n. For example, the a(3) = 1 through a(11) = 15 triples are:

  111   112   113   114   115   116   117   118   119

        121   122   141   133   161   144   181   155

        211   131   222   151   224   171   226   191

              212   411   223   233   225   244   227

              221         232   242   252   262   272

              311         313   323   333   334   335

                          322   332   414   343   344

                          331   422   441   424   353

                          511   611   522   433   434

                                      711   442   443

                                            622   515

                                            811   533

                                                  551

                                                  722

                                                  911

(End)

MATHEMATICA

Table[Length[Join@@Permutations/@Select[IntegerPartitions[n, {3}], !UnsameQ@@#&]], {n, 0, 100}] (* Gus Wiseman, Nov 11 2020 *)

CROSSREFS

Column k=3 of A242887.

A235451 counts 3-part compositions with distinct run-lengths

A001399(n-6) counts 3-part compositions in the complement.

A014311 intersected with A335488 ranks these compositions.

A140106 is the unordered case, with Heinz numbers A285508.

A261982 counts non-strict compositions of any length.

A001523 counts unimodal compositions, with complement A115981.

A007318 and A097805 count compositions by length.

A032020 counts strict compositions.

A047967 counts non-strict partitions, with Heinz numbers A013929.

A242771 counts triples that are not strictly increasing.

Cf. A000212, A000217, A001840, A128422, A156040, A332834, A337461, A337484, A337603, A337604.

Sequence in context: A021278 A336761 A143940 * A083349 A065230 A316478

Adjacent sequences:  A321770 A321771 A321772 * A321774 A321775 A321776

KEYWORD

nonn

AUTHOR

Alois P. Heinz, Nov 18 2018

STATUS

approved

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Last modified June 25 04:52 EDT 2021. Contains 345452 sequences. (Running on oeis4.)