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## What is the Gregory–Newton Interpolation Formula?

The **Gregory–Newton interpolation formula** is defined as

Where:

The Taylor series is the limiting case of the formula as c → 0 [1].

## Using the Formula to Find Function Values

The formula is perhaps best known for its ability to calculate a function value at an arbitrary point from the value (a + *h*) at an arithmetic sequence of points a, a + b, a + 2b,….

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,… [2]. 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) [3].

## Gregory–Newton Interpolation Formula for Integration

You can also use the **same formula** to approximate integrals [2]. 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.

## History

Both Newton and Gregory were inspired by Wallis’s loose heuristic method of interpolation [2]. 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 [2].

## References

[1] Vacher, H. Computational Geology 16: The Taylor Series and Error Propagation. Journal of Geoscience Education, v. 49, n.3, p. 305-313, May 2001 (edits, June 2005).

[2] Kline, M. (1990). Mathematical Thought from Ancient to Modern Times: Volume 2. OUP USA.

[3] Stillwell, J. Mathematics and Its History. 3rd Edition. Springer.

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