Richardson’s extrapolation.

Define for the mid-point trap rule

or for the end point rule

in either case to lowest order

so evaluate with h₁ and then with h₂ and eliminate a to find

so that

Using terms up to h⁴

With h₁ =1 and h₂ = 1/2

See also..\Fittery\nlfit-r\StdDev\3ptLinFit.docx