1,4

Also number of partitions of n-1 with a distinguished part different from all the others. [Comment corrected by Emeric Deutsch, Aug 13 2008]

In general the number of times that j appears in the partitions of n equals Sum_{k<n, k = n (mod j)} P(k). In particular this gives a formula for a(n), A024787, ..., A024794, for j = 2,...,10; it generalizes the formula given for A000070 for j=1. - Jose Luis Arregui (arregui(AT)posta.unizar.es), Apr 05 2002

Equals row sums of triangle A173238. - Gary W. Adamson, Feb 13 2010

The sums of two successive terms give A000070. - Omar E. Pol, Jul 12 2012

a(n) is also the difference between the sum of second largest and the sum of third largest elements in all partitions of n. More generally, the number of occurrences of k in all partitions of n equals the difference between the sum of k-th largest and the sum of (k+1)st largest elements in all partitions of n. And more generally, the sum of the number of occurrences of k, k+1, k+2..k+m in all partitions of n equals the difference between the sum of k-th largest and the sum of (k+m+1)st largest elements in all partitions of n. - Omar E. Pol, Oct 25 2012

Number of singletons in all partitions of n-1. A singleton in a partition is a part that occurs exactly once. Example: a(5) = 4 because in the partitions of 4, namely [1,1,1,1], [1,1,2'], [2,2], [1',3'], [4'] we have 4 singletons (marked by '). - Emeric Deutsch, Sep 12 2016

a(n) is also the number of non-isomorphic vertex-transitive cover graphs of lattice quotients of essential lattice congruences of the weak order on the symmetric group S_{n-1}. See Table 1 in the Hoang/Mütze reference in the Links section. - Torsten Muetze, Nov 28 2019

Assuming a partition is in weakly decreasing order, a(n) is also the number of times -1 occurs in the differences of the partitions of n+1. - George Beck, Mar 28 2023

a(n+2) is the number of nonzero terms in the polysymmetric signed elementary expansion of the polysymmetric unsigned elementary function E^+_n. - Aditya Khanna, Nov 26 2025

J. Riordan, Combinatorial Identities, Wiley, 1968, p. 184.

Alois P. Heinz and Vaclav Kotesovec, Table of n, a(n) for n = 1..10000 (terms 1..1000 from Alois P. Heinz)

Cristina Ballantine, Shaheen Nazir, Bridget Eileen Tenner, Karlee Westrem, and Chenchen Zhao, On partitions associated with elementary symmetric polynomials, arXiv:2510.01100 [math.CO], 2025.

David Benson, Radha Kessar, and Markus Linckelmann, Hochschild cohomology of symmetric groups in low degrees, arXiv:2204.09970 [math.GR], 2022.

Philip Cuthbertson, Fixed hooks in arbitrary columns of partitions, Integers (2025) Vol. 25, Art. No. A28. See p. 3.

Manosij Ghosh Dastidar and Sourav Sen Gupta, Generalization of a few results in Integer Partitions, arXiv preprint arXiv:1111.0094 [cs.DM], 2011.

Emeric Deutsch et al., Problem 11237, Amer. Math. Monthly, 115 (No. 7, 2008), 666-667. [From Emeric Deutsch, Aug 13 2008]

Hung Phuc Hoang and Torsten Mütze, Combinatorial generation via permutation languages. II. Lattice congruences, arXiv:1911.12078 [math.CO], 2019.

Aditya Khanna, Transition Matrices between Plethystic Bases of Polysymmetric Functions via Bijective Methods, arXiv:2510.12723 [math.CO], 2025. See pp. 5, 45.

Joseph Vandehey, Digital problems in the theory of partitions, Integers (2024) Vol. 24A, Art. No. A18. See p. 3.

a(n) = Sum_{k=1..floor(n/2)} A000041(n-2k). - Christian G. Bower, Jun 22 2000

a(n) = Sum_{k<n, k = n (mod 2)} P(k), P(k) = number of partitions of k as in A000041, P(0) = 1. - Jose Luis Arregui (arregui(AT)posta.unizar.es), Apr 05 2002

G.f.: (x^2/((1-x)*(1-x^2)^2))*Product_{j>=3} 1/(1-x^j) from Riordan reference second term, last eq.

a(n) = A006128(n-1) - A194452(n-1). - Omar E. Pol, Nov 20 2011

a(n) = A181187(n,2) - A181187(n,3). - Omar E. Pol, Oct 25 2012

a(n) ~ exp(Pi*sqrt(2*n/3)) / (2^(5/2) * Pi * sqrt(n)) * (1 - 25*Pi/(24*sqrt(6*n)) + (25/48 + 433*Pi^2/6912)/n). - Vaclav Kotesovec, Mar 07 2016, extended Nov 05 2016

a(n) = Sum_{k} k * A116595(n-1,k). - Emeric Deutsch, Sep 12 2016

G.f.: x^2/((1 - x)*(1 - x^2)) * Sum_{n >= 0} x^(2*n)/( Product_{k = 1..n} 1 - x^k ); that is, convolution of A004526 (partitions into 2 parts, or, modulo offset differences, partitions into parts <= 2) and A002865 (partitions into parts >= 2). - Peter Bala, Jan 17 2021

From Omar E. Pol, Oct 25 2012: (Start)

For n = 7 we have:

--------------------------------------

. Number

Partitions of 7 of 2's

--------------------------------------

7 .............................. 0

4 + 3 .......................... 0

5 + 2 .......................... 1

3 + 2 + 2 ...................... 2

6 + 1 .......................... 0

3 + 3 + 1 ...................... 0

4 + 2 + 1 ...................... 1

2 + 2 + 2 + 1 .................. 3

5 + 1 + 1 ...................... 0

3 + 2 + 1 + 1 .................. 1

4 + 1 + 1 + 1 .................. 0

2 + 2 + 1 + 1 + 1 .............. 2

3 + 1 + 1 + 1 + 1 .............. 0

2 + 1 + 1 + 1 + 1 + 1 .......... 1

1 + 1 + 1 + 1 + 1 + 1 + 1 ...... 0

------------------------------------

. 24 - 13 = 11

.

The difference between the sum of the second column and the sum of the third column of the set of partitions of 7 is 24 - 13 = 11 and equals the number of 2's in all partitions of 7, so a(7) = 11.

(End)

b:= proc(n, i) option remember; local f, g;

if n=0 or i=1 then [1, 0]

else f:= b(n, i-1); g:= `if`(i>n, [0$2], b(n-i, i));

[f[1]+g[1], f[2]+g[2]+`if`(i=2, g[1], 0)]

fi

end:

a:= n-> b(n, n)[2]:

seq(a(n), n=1..50); # Alois P. Heinz, May 18 2012

Table[ Count[ Flatten[ IntegerPartitions[n]], 2], {n, 1, 50} ]

(* Alternative: *)

b[n_, i_] := b[n, i] = Module[{f, g}, If[n==0 || i==1, {1, 0}, f = b[n, i - 1]; g = If[i>n, {0, 0}, b[n-i, i]]; {f[[1]] + g[[1]], f[[2]] + g[[2]] + If[i == 2, g[[1]], 0]}]]; a[n_] := b[n, n][[2]]; Table[a[n], {n, 1, 50}] (* Jean-François Alcover, Sep 22 2015, after Alois P. Heinz *)

Join[{0}, (1/((1 - x^2) QPochhammer[x]) + O[x]^50)[[3]]] (* Vladimir Reshetnikov, Nov 22 2016 *)

Table[Sum[(1 + (-1)^k)/2 * PartitionsP[n-k], {k, 2, n}], {n, 1, 50}] (* Vaclav Kotesovec, Aug 27 2017 *)

(Python)

from sympy import npartitions

def A024786(n): return sum(npartitions(n-(k<<1)) for k in range(1, (n>>1)+1)) # Chai Wah Wu, Oct 25 2023

Cf. A066633, A024787, A024788, A024789, A024790, A024791, A024792, A024793, A024794, A173238.

Column 2 of A060244.

First differences of A000097.

Sequence in context: A212548 A212549 A212550 * A385011 A299069 A097497

Adjacent sequences: A024783 A024784 A024785 * A024787 A024788 A024789

approved