Fractions

Three constructions for putting expressions above others are

frac:-

$\frac{1}{(x + 3)}$ produces $\frac{1}{(x + 3)}$.

choose:-

${n + 1 \choose 3}$ produces ${n + 1 \choose 3}$.

atop:-

${x \atop y}$ produces ${x \atop y}$.

These constructions can be used with ones described earlier. E.g.,

\[ \sum_{-1\le i \le 1 \atop 0 < j < \infty} f(i,j)\]

gives

$$\sum_{-1\le i \le 1 \atop 0 < j < \infty} f(i,j)$$