Pieri formula

Statement
The Pieri formula is a formula about the Schur elements in the tableau ring. It is a particular case of the Littlewood-Richardson formula, and can also be thought of as an assertion about the values of Littlewood-Richardson numbers.

Statement in the tableau ring

 * $$S_\lambda.S_{(p)} = \sum_{\mu}S_\mu$$ where $$\mu$$ ranges over all partitions obtained by adding $$p$$ boxes to $$\lambda$$, no two in the same column
 * $$S_\lambda.S_{(1^p)} = \sum_{\mu} S_\mu$$ where $$\mu$$ ranges over all partitions obtained by adding $$p$$ boxes to $$\lambda$$, no two in the same row

Statement in terms of Littlewood-Richardsom numbers
The Littlewood-Richardson formula gives a general way of multiplying any two Schur elements in the tableau ring:

$$S_\lambda.S_\mu = \sum_\nu c_{\lambda\mu}^\nu S_\nu$$

where $$c_{\lambda\mu}^\nu$$ are certain combinatorial quantities associated with the triple of partitions $$\lambda, \mu, \nu$$. The Pieri formula gives the special cases of this where $$\mu = (p)$$ (the partition into one part) and $$\mu = (1^p)$$ (the partition into $$p$$ parts). What it tells us is that:


 * $$c_{\lambda(p)}^{\nu} = 1$$ if and only if $$\nu$$ is obtained by adding $$p$$ boxes to $$\lambda$$, no two in the same column. It is zero otherwise.


 * $$c_{\lambda(1^p)}^{\nu} = 1$$ if and only if $$\nu$$ is obtained by adding $$p$$ boxes to $$\lambda$$, no two in the same row. It is zero otherwise.

Statement in terms of Schur polynomials
By the canonical homomorphism from the tableau ring to the polynomial ring, which sends the Schur element $$S_\lambda$$ to the polynomial $$s_\lambda$$, we get:


 * $$s_\lambda.s_{(p)} = \sum_\mu s_\mu$$ where $$\mu$$ ranges over all partitions obtained by adding $$p$$ boxes to $$\lambda$$, no two in the same column
 * $$s_\lambda.s_{(1^p)} = \sum_{\mu} s_\mu$$ where $$\mu$$ ranges over all partitions obtained by adding $$p$$ boxes to $$\lambda$$, no two in the same row

Notice that $$s_{(p)}$$ is the same as $$h_p$$ (the complete symmetric polynomial of degree $$p$$). Similarly, $$s_{(1^p)}$$ is the same as $$e_p$$ (the elementary symmetric polynomial of degree $$p$$). This gives us:


 * $$s_\lambda.h_p = \sum_\mu s_\mu$$ where $$\mu$$ ranges over all partitions obtained by adding $$p$$ boxes to $$\lambda$$, no two in the same column
 * $$s_\lambda.e_p = \sum_{\mu} s_\mu$$ where $$\mu$$ ranges over all partitions obtained by adding $$p$$ boxes to $$\lambda$$, no two in the same row

Corresponding statement for cohomology classes
The Pieri formula also holds when we interpret $$S_\lambda$$, not as the Schur element for the partition $$\lambda$$, but rather, as the cohomology class of the Schubert variety corresponding to the partition $$\lambda$$.