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Smallest Parts Function


The smallest parts function spt(n) counts, with multiplicity, the occurrences of the smallest part in all partitions of a positive integer n. For example, the partitions of 4 contribute 4+2+2+1+1=10. For n=1, 2, ..., the values are 1, 3, 5, 10, 14, 26, 35, 57, 80, 119, ... (OEIS A092269). Set spt(0)=0.

For a prime number l>=5, let b_l be the integer with 1<=b_l<l and 24b_l=1  (modl). A canonical congruence prime is one for which

 spt(ln+b_l)=0  (modl)

for every nonnegative integer n. Rolen (2026) proved that these primes are precisely 5, 7, and 13. The corresponding progressions are 5n+4, 7n+5, and 13n+6. This classifies the canonical prime progressions, not all possible congruences for the function.

Rolen (2026) reports AI assistance in developing the proof and an AxiomProver formalization in Lean.


See also

Partition

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References

Rolen, L. "Class Polynomial Obstructions to Canonical Congruences for the Smallest Parts Function." 19 Oct 2026. https://arxiv.org/abs/2610.27145.Sloane, N. J. A. Sequence A092269 in "The On-Line Encyclopedia of Integer Sequences."

Cite this as:

Weisstein, Eric W. "Smallest Parts Function." From MathWorld--A Wolfram Resource. https://mathworld.wolfram.com/SmallestPartsFunction.html

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