Legendre polynomials 1.0

The built-in legendre() calculates the Legendre polynomials calculated ALL the orders for a given degree. If you only need a given order, this is a waste of memory and computing time (especially for large blocks of data). The function legendreP(l,m,x) is a drop-in substitute for legendre(l,x), except that it only calculates the required order.The polynomial coefficients are calculated analytically from the Rodrigues' formula, To further optimize the computation of coefficients (significant for small data sets), the coefficients are computed recursively from earlier coefficients (to avoid computing multiple factorials). The polynomial is computed using exactly l/2 multiplications and additions for any structure of the data array x.The polynomial coefficients for first 3 degrees were checked manually against the exact values. The numerical returns were checked for every order for all degrees up to m using an array of size 128^1, filled with random numbers between -1 and 1; the difference compared with the built-in were within numerical errors (relative error ~1e-15). Some of the orders were checked for all degrees up to l=20 as well.This function is much faster than the built in, especially for larger degrees; for 100^3 array, the built-in legendre(8,x) takes about 9.5 sec, while legendreP(8,6,x) takes only 0.15sec. Even calculating all 9 orders takes only about 0.15 sec.There is no error checking (e.g., 0

Requirements:	No special requirements
Platforms:	Matlab
Keyword:	Built,  Compared,  Difference,  Error,  Errors,  Exact,  Faster,  L3d20,  Larger,  Legendre,  Numerical,  Relative,  Returns,  Takes,  Values,  Wellthis
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