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Number of integer compositions of n whose leaders of maximal strictly increasing runs sum to 2.
2

%I #11 Aug 15 2024 02:04:32

%S 0,0,2,0,2,3,4,7,8,14,17,27,33,48,63,84,112,147,191,248,322,409,527,

%T 666,845,1062,1336,1666,2079,2579,3190,3936,4842,5933,7259,8854,10768,

%U 13074,15826,19120,23048,27728,33279,39879,47686,56916,67818,80667,95777,113552,134396

%N Number of integer compositions of n whose leaders of maximal strictly increasing runs sum to 2.

%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.

%H Andrew Howroyd, <a href="/A374705/b374705.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.: (x*Q(x)/(1 + x))^2 + x^2*Q(x)/((1 + x)*(1 + x^2)), where Q(x) is the g.f. of A000009. - _Andrew Howroyd_, Aug 14 2024

%e The a(0) = 0 through a(9) = 14 compositions:

%e . . (2) . (112) (23) (24) (25) (26) (27)

%e (11) (121) (113) (114) (115) (116) (117)

%e (131) (141) (151) (161) (171)

%e (1212) (1123) (1124) (234)

%e (1213) (1214) (1125)

%e (1231) (1241) (1134)

%e (1312) (1313) (1215)

%e (1412) (1251)

%e (1314)

%e (1341)

%e (1413)

%e (1512)

%e (12123)

%e (12312)

%t Table[Length[Select[Join@@Permutations /@ IntegerPartitions[n],Total[First/@Split[#,Less]]==2&]],{n,0,15}]

%o (PARI) seq(n)={my(A=O(x^(n-1)), q=eta(x^2 + A)/eta(x + A)); Vec((q*x/(1 + x))^2 + q*x^2/((1 + x)*(1 + x^2)), -n-1)} \\ _Andrew Howroyd_, Aug 14 2024

%Y For leaders of weakly decreasing runs we have A004526.

%Y The case of strict compositions is A096749.

%Y For leaders of anti-runs we have column k = 2 of A374521.

%Y Leaders of strictly increasing runs in standard compositions are A374683.

%Y Ranked by positions of 2s in A374684.

%Y Column k = 2 of A374700.

%Y A003242 counts anti-run compositions.

%Y A011782 counts compositions.

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

%Y A274174 counts contiguous compositions, ranks A374249.

%Y Cf. A000009, A096765, A106356, A124766, A238343, A261982, A333213.

%K nonn

%O 0,3

%A _Gus Wiseman_, Aug 12 2024

%E a(26) onwards from _Andrew Howroyd_, Aug 14 2024