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A332339
Number of alternately co-strong reversed integer partitions of n.
9
1, 1, 2, 3, 4, 5, 8, 8, 12, 14, 18, 20, 29, 28, 40, 45, 54, 59, 82, 81, 108, 118, 141, 154, 204, 204, 255, 285, 339, 363, 458, 471, 580, 632, 741, 806, 983, 1015, 1225, 1341, 1562, 1667, 2003, 2107, 2491, 2712, 3101, 3344, 3962, 4182, 4860, 5270, 6022, 6482
OFFSET
0,3
COMMENTS
A sequence is alternately co-strong if either it is empty, equal to (1), or its run-lengths are weakly increasing (co-strong) and, when reversed, are themselves an alternately co-strong sequence.
Also the number of alternately strong integer partitions of n.
EXAMPLE
The a(1) = 1 through a(8) = 12 reversed partitions:
(1) (2) (3) (4) (5) (6) (7) (8)
(11) (12) (13) (14) (15) (16) (17)
(111) (22) (23) (24) (25) (26)
(1111) (122) (33) (34) (35)
(11111) (123) (124) (44)
(222) (133) (125)
(1122) (1222) (134)
(111111) (1111111) (233)
(1133)
(2222)
(11222)
(11111111)
For example, starting with the composition y = (1,2,3,3,4,4,4) and repeatedly taking run-lengths and reversing gives (1,2,3,3,4,4,4) -> (3,2,1,1) -> (2,1,1) -> (2,1) -> (1,1) -> (2) -> (1). All of these have weakly increasing run-lengths and the last is equal to (1), so y is counted under a(21).
MATHEMATICA
tniQ[q_]:=Or[q=={}, q=={1}, And[LessEqual@@Length/@Split[q], tniQ[Reverse[Length/@Split[q]]]]];
Table[Length[Select[Sort/@IntegerPartitions[n], tniQ]], {n, 0, 30}]
CROSSREFS
The total (instead of alternating) version is A316496.
Alternately strong partitions are A317256.
The case of ordinary (not reversed) partitions is (also) A317256.
The generalization to compositions is A332338.
Sequence in context: A325415 A331076 A316496 * A100882 A171979 A361858
KEYWORD
nonn
AUTHOR
Gus Wiseman, Feb 17 2020
STATUS
approved