A monomial is a polynomial wich has only one term with `1` as coefficient. The degree of a monomial is the sum of degrees of all the variables.
For example, `x^2yz^3` is a monomial of degree `6`.
`a(n)` is the number of monomials of degree at most `n` in `n` variables.
For example, `a(2)=6` because we have monomials `1`, `x_1`, `x_2`, `x_1x_2`, `x_1^2` and `x_2^2`.