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Number of integer compositions of n whose leaders of strictly increasing runs are strictly decreasing.
15

%I #19 Jul 31 2024 22:11:41

%S 1,1,1,3,3,6,10,13,21,32,48,66,101,144,207,298,415,592,833,1163,1615,

%T 2247,3088,4259,5845,7977,10862,14752,19969,26941,36310,48725,65279,

%U 87228,116274,154660,205305,271879,359400,474157,624257,820450,1076357,1409598

%N Number of integer compositions of n whose leaders of strictly increasing runs are strictly decreasing.

%C The leaders of strictly increasing runs in a sequence are obtained by splitting it into maximal strictly increasing subsequences and taking the first term of each.

%C Also the number of ways to choose a strict integer partition of each part of an integer composition of n (A304969) such that the minima are strictly decreasing. The weakly decreasing version is A374697.

%H Andrew Howroyd, <a href="/A374689/b374689.txt">Table of n, a(n) for n = 0..1000</a>

%H Gus Wiseman, <a href="/A374629/a374629.txt">Sequences counting and ranking compositions by their leaders (for six types of runs)</a>.

%F G.f.: Product_{i>0} (1 + (x^i)*Product_{j>i} (1 + x^j)). - _John Tyler Rascoe_, Jul 29 2024

%e The a(0) = 1 through a(8) = 21 compositions:

%e () (1) (2) (3) (4) (5) (6) (7) (8)

%e (12) (13) (14) (15) (16) (17)

%e (21) (31) (23) (24) (25) (26)

%e (32) (42) (34) (35)

%e (41) (51) (43) (53)

%e (212) (123) (52) (62)

%e (213) (61) (71)

%e (231) (124) (125)

%e (312) (214) (134)

%e (321) (241) (215)

%e (313) (251)

%e (412) (314)

%e (421) (323)

%e (341)

%e (413)

%e (431)

%e (512)

%e (521)

%e (2123)

%e (2312)

%e (3212)

%t Table[Length[Select[Join@@Permutations /@ IntegerPartitions[n],Greater@@First/@Split[#,Less]&]],{n,0,15}]

%o (PARI)

%o C_x(N) = {my(x='x+O('x^N), h=prod(i=1,N, 1+(x^i)*prod(j=i+1,N, 1+x^j))); Vec(h)}

%o C_x(50) \\ _John Tyler Rascoe_, Jul 29 2024

%Y The weak version appears to be A189076.

%Y Ranked by positions of strictly decreasing rows in A374683.

%Y The opposite version is A374762.

%Y Types of runs (instead of strictly increasing):

%Y - For leaders of identical runs we have A000041.

%Y - For leaders of anti-runs we have A374680.

%Y - For leaders of weakly increasing runs we have A188920.

%Y - For leaders of weakly decreasing runs we have A374746.

%Y - For leaders of strictly decreasing runs we have A374763.

%Y Types of run-leaders (instead of strictly decreasing):

%Y - For identical leaders we have A374686, ranks A374685.

%Y - For distinct leaders we have A374687, ranks A374698.

%Y - For strictly increasing leaders we have A374688.

%Y - For weakly increasing leaders we have A374690.

%Y - For weakly decreasing leaders we have A374697.

%Y A003242 counts anti-run compositions, ranks A333489.

%Y A011782 counts compositions.

%Y A238130, A238279, A333755 count compositions by number of runs.

%Y A335456 counts patterns matched by compositions.

%Y A373949 counts compositions by run-compressed sum, opposite A373951.

%Y A374700 counts compositions by sum of leaders of strictly increasing runs.

%Y Cf. A000009, A106356, A238343, A261982, A304969, A333213, A374632, A374635, A374640, A374679.

%K nonn

%O 0,4

%A _Gus Wiseman_, Jul 27 2024

%E a(26) onwards from _John Tyler Rascoe_, Jul 29 2024