The spt function (smallest parts function) is a function in number theory that counts the sum of the number of smallest parts in each integer partition of a positive integer. It is related to the partition function.[1]

The first few values of spt(n) are:

1, 3, 5, 10, 14, 26, 35, 57, 80, 119, 161, 238, 315, 440, 589 ... (sequence A092269 in the OEIS)

Example

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For example, there are five partitions of 4 (with smallest parts underlined):

4
3 + 1
2 + 2
2 + 1 + 1
1 + 1 + 1 + 1

These partitions have 1, 1, 2, 2, and 4 smallest parts, respectively. So spt(4) = 1 + 1 + 2 + 2 + 4 = 10.

Properties

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Like the partition function, spt(n) has a generating function. It is given by

 

where  .

The function   is related to a mock modular form. Let   denote the weight 2 quasi-modular Eisenstein series and let   denote the Dedekind eta function. Then for  , the function

 

is a mock modular form of weight 3/2 on the full modular group   with multiplier system  , where   is the multiplier system for  .

While a closed formula is not known for spt(n), there are Ramanujan-like congruences including

 
 
 

References

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  1. ^ Andrews, George E. (2008-11-01). "The number of smallest parts in the partitions of n". Journal für die Reine und Angewandte Mathematik. 2008 (624): 133–142. doi:10.1515/CRELLE.2008.083. ISSN 1435-5345. S2CID 123142859.