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Triangle read by rows where T(n,k) is the number of integer partitions of n with reverse-alternating sum k ranging from -n to n in steps of 2.
109

%I #14 Jan 06 2024 14:32:07

%S 1,0,1,0,1,1,0,1,1,1,0,1,2,1,1,0,1,2,2,1,1,0,1,2,3,3,1,1,0,1,2,4,3,3,

%T 1,1,0,1,2,4,5,5,3,1,1,0,1,2,4,7,5,6,3,1,1,0,1,2,4,8,7,9,6,3,1,1,0,1,

%U 2,4,8,12,7,11,6,3,1,1,0,1,2,4,8,14,11,14,12,6,3,1,1

%N Triangle read by rows where T(n,k) is the number of integer partitions of n with reverse-alternating sum k ranging from -n to n in steps of 2.

%C The reverse-alternating sum of a partition (y_1,...,y_k) is Sum_i (-1)^(k-i) y_i. This is also (-1)^(k-1) times the sum of the even-indexed parts minus the sum of the odd-indexed parts.

%C Also the number of reversed integer partitions of n with alternating sum k ranging from -n to n in steps of 2.

%C Also the number of integer partitions of n with (-1)^(m-1) * b = k where m is the greatest part and b is the number of odd parts, with k ranging from -n to n in steps of 2.

%H Andrew Howroyd, <a href="/A344612/b344612.txt">Table of n, a(n) for n = 0..1325</a> (rows 0..50)

%e Triangle begins:

%e 1

%e 0 1

%e 0 1 1

%e 0 1 1 1

%e 0 1 2 1 1

%e 0 1 2 2 1 1

%e 0 1 2 3 3 1 1

%e 0 1 2 4 3 3 1 1

%e 0 1 2 4 5 5 3 1 1

%e 0 1 2 4 7 5 6 3 1 1

%e 0 1 2 4 8 7 9 6 3 1 1

%e 0 1 2 4 8 12 7 11 6 3 1 1

%e 0 1 2 4 8 14 11 14 12 6 3 1 1

%e 0 1 2 4 8 15 19 11 18 12 6 3 1 1

%e 0 1 2 4 8 15 24 15 23 20 12 6 3 1 1

%e 0 1 2 4 8 15 26 30 15 31 21 12 6 3 1 1

%e For example, row n = 7 counts the following partitions:

%e (61) (52) (43) (331) (322) (511) (7)

%e (4111) (2221) (22111) (421)

%e (3211) (1111111) (31111)

%e (211111)

%e Row n = 9 counts the following partitions:

%e 81 72 63 54 441 333 522 711 9

%e 6111 4221 3222 22221 432 621

%e 5211 3321 33111 531 51111

%e 411111 4311 2211111 32211

%e 222111 111111111 42111

%e 321111 3111111

%e 21111111

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

%t Table[Length[Select[IntegerPartitions[n],sats[#]==k&]],{n,0,15},{k,-n,n,2}]

%o (PARI) row(n)={my(v=vector(n+1)); forpart(p=n, my(s=-sum(i=1, #p, p[i]*(-1)^i)); v[(s+n)/2+1]++); v} \\ _Andrew Howroyd_, Jan 06 2024

%Y Row sums are A000041.

%Y The midline k = n/2 is also A000041.

%Y The right half (i.e., k >= 0) for even n is A344610.

%Y The rows appear to converge to A344611 (from left) and A006330 (from right).

%Y The non-reversed version is A344651 (A239830 interleaved with A239829).

%Y The strict version is A344739.

%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 A120452 counts partitions of 2n with rev-alt sum 2 (negative: A344741).

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

%Y A325534/A325535 count separable/inseparable partitions.

%Y A344618 gives reverse-alternating sums of standard compositions.

%Y Cf. A000070, A000097, A003242, A027187, A124754, A152146, A344607, A344608, A344649, A344650, A344654.

%K nonn,tabl

%O 0,13

%A _Gus Wiseman_, Jun 01 2021