What is the Gregory–Newton Interpolation Formula?
Using the Formula to Find Function Values
To calculate a value for f(x), for any x between known values, replace h in the formula with x – a. This isn’t necessarily the true function value, but rather a value for a polynomial in h that equals the function values at a, a + c, a + 2c,… . Therefore, the formula is valid for any function that is the limit of its own polynomial approximation (i.e., any functions that can be represented by a power series) .
Gregory–Newton Interpolation Formula for Integration
You can also use the same formula to approximate integrals . Let’s say you had a function g(x) that you want to integrate:
- Use g(x) values to get g(a), g(a + c), g(a + 2c),…
- Find the differences of the values you obtained in Step 1, along with their higher-order differences.
- Substitute your solutions from Step 2 into the formula.
These steps will give you a polynomial approximation for your function, which can be integrated easily—giving you your approximate integral.
Both Newton and Gregory were inspired by Wallis’s loose heuristic method of interpolation . Gregory discovered the general formula first, followed by an independent discovery by Newton. Both mathematicians used to formula to derive the binomial theorem. However, while Newton sketched a proof for the formula, Gregory did not .
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