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  "body": "\n## Summary\n\nThe backward difference operator $\\nabla_h$ sends a function $f$ to the function $x \\mapsto f(x) - f(x - h)$.\nIt looks one step back instead of one step ahead.\nIt is natural in time-stepping and interpolation schemes that use only values already computed.\n\n## Explanations\n\n### L1 human\n\nThe backward difference compares the value now with the value one step earlier.\nIf $u_n$ is the balance of an account at the end of month $n$, then $\\nabla u_n = u_n - u_{n-1}$ is the change during month $n$.\nFor the squares $0, 1, 4, 9, 16$ the backward differences at $n = 1, 2, 3, 4$ are $1, 3, 5, 7$.\nIt is the forward difference read from the other end of the step: $\\nabla_h f(x) = \\Delta_h f(x - h)$.\n",
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