 # Trinomial Coefficient & Theorem: Definition

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In probability, the trinomial coefficient (sometimes called the central trinomial coefficient) is the number of ways of partitioning a set of objects into three disjoint subsets.

The value of the trinomial coefficient can be calculated as (Hilton et al., 2002; Kuri-Morales & Simari, 2002): Where:

• ! is a factorial,
• k is the number of objects you want to partition (non-negative integer),
• a, b, c, are the three subsets (also must be non-negative integers),

## Trinomial Coefficient and the Trinomial Theorem

Trinomial coefficients appear in the trinomial theorem, which expands a trinomial (x + y + z)k as the algebraic coefficient of xa yb zc: ## Relation to Binomial Coefficient

The trinomial coefficient is a close relative of the binomial coefficient:

• The trinomial coefficient appears in the expansion of a trinomial (x + y + z)k and is the number of ways of partitioning three sets.
• The binomial coefficient appears in the expansion of a binomial (x + y)k, and is the number of ways of partitioning two sets.

## The Trinomial Triangle

The trinomial triangle, an extension of Pascal’s triangle, gives the coefficients of the expansion (1 + x + x2)k. The entries in each row represent “k”. For example, the second row (k = 2) has entries 1 2 3 2 1, giving the expansion
(1 + x + x2)2 = 1 + 2x + 3x2 + 2x3 + x4.

## References

Engelward, A. The Analogies between Binomial and Trinomial Coefficients. Retrieved September 29, 2020 from: http://people.math.harvard.edu/~engelwar/MathS305/Trinomial%20Coefficients.pdf
Graham, D. & Allinson, N. (1998). Characterizing Virtual Eigensignatures for General Purpose Face Recognition. Face Recognition From Theory to Application 163, 446-456.
Hilton, P. et al. (2002). Mathematical Vistas: From a Room with Many Windows. Springer.
Kuri-Morales, A. & Simari, G. (2010). Advances in Artificial Intelligence – IBERAMIA 2010. 12th Ibero-American Conference on AI, Bahía Blanca, Argentina, November 1-5, 2010, Proceedings. Springer Berlin Heidelberg.

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Stephanie Glen. "Trinomial Coefficient & Theorem: Definition" From StatisticsHowTo.com: Elementary Statistics for the rest of us! https://www.statisticshowto.com/trinomial-coefficient-theorem-definition/
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