Compute the forward difference `f[x + h] - f[x]` of an expression.
DifferenceDelta[f, i]
DifferenceDelta[f, {i, n}]
DifferenceDelta[f, {i, n, h}]
DifferenceDelta[f, i, j, …]
DifferenceDelta[x^2, x]
→ 1 + 2*xDifferenceDelta[1/(2 n + 1), n]
→ -2/((1 + 2*n)*(3 + 2*n))DifferenceDelta[x^2 + x, x]
→ 2*(1 + x)