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Number of integer partitions of 2n with reverse-alternating sum -2.
22

%I #10 Jun 12 2021 06:05:02

%S 0,0,1,2,4,8,14,24,39,62,95,144,212,309,442,626,873,1209,1653,2245,

%T 3019,4035,5348,7051,9229,12022,15565,20063,25722,32847,41746,52862,

%U 66657,83768,104873,130889,162797,201902,249620,307789,378428,464122,567721,692828,843448

%N Number of integer partitions of 2n with reverse-alternating sum -2.

%C The reverse-alternating sum of a partition (y_1,...,y_k) is Sum_i (-1)^(k-i) y_i. This is equal to (-1)^(r-1) times the number of odd parts, where r is the greatest part, so a(n) is the number of integer partitions of 2n with exactly two odd parts, neither of which is the greatest.

%C Also the number of reversed integer partitions of 2n with alternating sum -2.

%e The a(2) = 1 through a(6) = 14 partitions:

%e (31) (42) (53) (64) (75)

%e (3111) (3221) (3331) (4332)

%e (4211) (4222) (4431)

%e (311111) (4321) (5322)

%e (5311) (5421)

%e (322111) (6411)

%e (421111) (322221)

%e (31111111) (333111)

%e (422211)

%e (432111)

%e (531111)

%e (32211111)

%e (42111111)

%e (3111111111)

%t sats[y_]:=Sum[(-1)^(i-Length[y])*y[[i]],{i,Length[y]}];

%t Table[Length[Select[IntegerPartitions[n],sats[#]==-2&]],{n,0,30,2}]

%t - or -

%t Table[Length[Select[IntegerPartitions[n],EvenQ[Max[#]]&&Count[#,_?OddQ]==2&]],{n,0,30,2}]

%Y The version for -1 instead of -2 is A000070.

%Y The non-reversed negative version is A000097.

%Y The ordered version appears to be A001700.

%Y The version for 1 instead of -2 is A035363.

%Y The whole set of partitions of 2n is counted by A058696.

%Y The strict case appears to be A065033.

%Y The version for -1 instead of -2 is A306145.

%Y The version for 2 instead of -2 is A344613.

%Y A000041 counts partitions of 2n with alternating sum 0, ranked by A000290.

%Y A103919 counts partitions by sum and alternating sum (reverse: A344612).

%Y A124754 gives alternating sums of standard compositions (reverse: A344618).

%Y A316524 is the alternating sum of the prime indices of n (reverse: A344616).

%Y A344610 counts partitions by sum and positive reverse-alternating sum.

%Y A344611 counts partitions of 2n with reverse-alternating sum >= 0.

%Y Cf. A001250, A003242, A006330, A027187, A028260, A344604, A344607, A344608, A344650, A344651, A344654, A344739.

%K nonn

%O 0,4

%A _Gus Wiseman_, Jun 08 2021

%E More terms from _Bert Dobbelaere_, Jun 12 2021