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Number of even-length integer partitions of 2n with integer mean.
3

%I #12 Mar 24 2023 17:58:56

%S 0,1,3,4,10,6,33,8,65,68,117,12,583,14,319,1078,1416,18,3341,20,8035,

%T 5799,1657,24,36708,16954,3496,24553,68528,30,192180,32,178802,91561,

%U 14625,485598,955142,38,29223,316085,2622697,42,3528870,44,2443527,5740043

%N Number of even-length integer partitions of 2n with integer mean.

%H Andrew Howroyd, <a href="/A361655/b361655.txt">Table of n, a(n) for n = 0..1000</a>

%e The a(0) = 0 through a(5) = 6 partitions:

%e . (11) (22) (33) (44) (55)

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

%e (1111) (51) (62) (73)

%e (111111) (71) (82)

%e (2222) (91)

%e (3221) (1111111111)

%e (3311)

%e (4211)

%e (5111)

%e (11111111)

%e For example, the partition (4,2,1,1) has length 4 and mean 2, so is counted under a(4).

%t Table[Length[Select[IntegerPartitions[2n], EvenQ[Length[#]]&&IntegerQ[Mean[#]]&]],{n,0,15}]

%o (PARI) a(n)=if(n==0, 0, sumdiv(n, d, polcoef(1/prod(k=1, 2*d, 1 - x^k + O(x*x^(2*(n-d)))), 2*(n-d)))) \\ _Andrew Howroyd_, Mar 24 2023

%Y Even-length partitions are counted by A027187, bisection A236913.

%Y Including odd-length partitions gives A067538 bisected, ranks A316413.

%Y For median instead of mean we have A361653.

%Y The odd-length version is counted by A361656.

%Y A000041 counts integer partitions, strict A000009.

%Y A051293 counts subsets with integer mean, median A000975.

%Y A058398 counts partitions by mean, see also A008284, A327482.

%Y A325347 counts partitions with integer median, complement A307683.

%Y A326567/A326568 gives mean of prime indices.

%Y A326622 counts factorizations with integer mean.

%Y Cf. A000016, A027193, A067659, A082550, A102627, A237984, A240219, A327475, A348551, A359893.

%K nonn

%O 0,3

%A _Gus Wiseman_, Mar 23 2023

%E Terms a(36) and beyond from _Andrew Howroyd_, Mar 24 2023