, also denoted (Abramowitz and Stegun 1972, p. 825), gives the number of ways of writing the integer as a sum of positive integers without regard to order with the constraint that all integers in a given partition are distinct. For example, , since the partitions of 10 into distinct parts are , , , , , , , , , . The function is implemented in the Wolfram Language as PartitionsQ[n]. is generally defined to be 1.
The values for , 2, ... are 1, 1, 2, 2, 3, 4, 5, 6, 8, 10, ... (OEIS A000009).
The first few prime values of are for indices 3, 4, 5, 7, 22, 70, 100, 495, 1247, 2072, 320397, 3335367, 16168775, 37472505, 52940251, 78840125, 81191852, ... (OEIS A035359), corresponding to values 2, 2, 3, 5, 89, 29927, 444793, 602644050950309, ... (OEIS A051005), with no others up to (M. Alekseyev, Jul. 10, 2015).
is also the number of partitions of with odd parts, sometimes denoted (Andrews 1998, p. 237).
The generating function for is
(1)
 
(2)
 
(3)
 
(4)
 
(5)
 
(6)

where and areqPochhammer symbols.
This can also be interpreted as another form of the Jacobi triple product, written in terms of the Qfunctions as
(7)

(Borwein and Borwein 1987, p. 64).
A recurrence relation is given by and
(8)

where
(9)

and
(10)

is the odd divisor function giving the sum of odd divisors of : 1, 1, 4, 1, 6, 4, 8, ... (OEIS A000593; Abramowitz and Stegun 1972, p. 826).
Another recurrence relation is given by and
(11)

where
(12)

(E. Georgiadis, A. V. Sutherland, and K. S. Kedlaya; Sloane).
satisfies the inequality
(13)

for . has the asymptotic series
(14)

(Abramowitz and Stegun 1972, p. 826).
A Rademacherlike convergent series for is given by
(15)

where
(16)

(P. J. Grabner, pers. comm., Sep. 10, 2003; Hagis 1964ab, 1965), where means and are relatively prime,
(17)

is a Dedekind sum, is the floor function, and is the zeroth order Bessel function of the first kind. Equation (16) corrects Abramowitz and Stegun (1972, p. 825), which erroneously state to be identical to the analogous expression in the formula for ). (15) can also be written explicitly as
(18)

where is a generalized hypergeometric function.
Let denote the number of ways of partitioning into exactly distinct parts. For example, since there are four partitions of 10 into three distinct parts: , , , and . is given by
(19)

where is the partition function P and is a binomial coefficient (Comtet 1974, p. 116). The following table gives the first few values of (OEIS A008289; Comtet 1974, pp. 115116).
1  2  3  4  
1  1  
2  1  
3  1  1  
4  1  1  
5  1  2  
6  1  2  1  
7  1  3  1  
8  1  3  2  
9  1  4  3  
10  1  4  4  1 