OFFSET
0,9
COMMENTS
A composition of n is a finite sequence of positive integers summing to n.
A sequence of integers is unimodal if it is the concatenation of a weakly increasing and a weakly decreasing sequence.
LINKS
Andrew Howroyd, Table of n, a(n) for n = 0..1325 (rows 0..50)
Eric Weisstein's World of Mathematics, Unimodal Sequence.
FORMULA
G.f.: A(x,y) = 1 + Sum_{j>0} y*x^j/((1 - y*x^j)*Product_{k>j} (1 - y*x^k)^2). - Andrew Howroyd, Jan 11 2024
EXAMPLE
Triangle begins:
1
0 1
0 1 1
0 1 2 1
0 1 3 2 1
0 1 4 5 2 1
0 1 5 7 5 2 1
0 1 6 11 10 5 2 1
0 1 7 15 16 10 5 2 1
0 1 8 20 24 20 10 5 2 1
0 1 9 25 36 31 20 10 5 2 1
0 1 10 32 50 50 36 20 10 5 2 1
0 1 11 38 67 73 59 36 20 10 5 2 1
Column n = 7 counts the following compositions:
(7) (16) (115) (1114) (11113) (111112) (1111111)
(25) (124) (1123) (11122) (211111)
(34) (133) (1222) (21112)
(43) (214) (2113) (22111)
(52) (223) (2122) (31111)
(61) (313) (2212)
(322) (2221)
(331) (3112)
(412) (3211)
(421) (4111)
(511)
MATHEMATICA
unimodQ[q_]:=Or[Length[q]<=1, If[q[[1]]<=q[[2]], unimodQ[Rest[q]], OrderedQ[Reverse[q]]]];
Table[Length[Select[Join@@Permutations/@IntegerPartitions[n, {k}], unimodQ[-#]&]], {n, 0, 10}, {k, 0, n}]
PROG
(PARI) T(n)={[Vecrev(p) | p<-Vec(1 + sum(j=1, n, y*x^j/((1-y*x^j) * prod(k=j+1, n-j, 1 - y*x^k + O(x*x^(n-j)))^2)))]}
{ my(A=T(10)); for(n=1, #A, print(A[n])) } \\ Andrew Howroyd, Jan 11 2024
CROSSREFS
The case of partitions is A072233.
Dominated by A072704 (the non-negated version).
The strict case is A072705.
The case of constant compositions is A113704.
Row sums are A332578.
Unimodal compositions are A001523.
Unimodal normal sequences appear to be A007052.
Non-unimodal compositions are A115981.
Non-unimodal normal sequences are A328509.
Numbers whose negated unsorted prime signature is not unimodal are A332282.
Partitions whose negated run-lengths are unimodal are A332638.
Compositions whose negation is not unimodal are A332669.
Partitions whose negated 0-appended first differences are unimodal: A332728.
KEYWORD
nonn,tabl
AUTHOR
Gus Wiseman, Feb 29 2020
STATUS
approved