# legendre_product_polynomial

legendre_product_polynomial, an Octave code which cdefines a Legendre product polynomial (LPP), creating a multivariate polynomial as the product of univariate Legendre polynomials.

The Legendre polynomials are a polynomial sequence L(I,X), with polynomial I having degree I.

The first few Legendre polynomials are

```        0: 1
1: x
2: 3/2 x^2 - 1/2
3: 5/2 x^3 - 3/2 x
4: 35/8 x^4 - 30/8 x^2 + 3/8
5: 63/8 x^5 - 70/8 x^3 + 15/8 x
```

A Legendre product polynomial may be defined in a space of M dimensions by choosing M indices. To evaluate the polynomial at a point X, compute the product of the corresponding Legendre polynomials, with each the I-th polynomial evaluated at the I-th coordinate:

```        L((I1,I2,...IM),X) = L(1,X(1)) * L(2,X(2)) * ... * L(M,X(M)).
```

Families of polynomials which are formed in this way can have useful properties for interpolation, derivable from the properties of the 1D family.

While it is useful to generate a Legendre product polynomial from its index set, and it is easy to evaluate it directly, the sum of two Legendre product polynomials cannot be reduced to a single Legendre product polynomial. Thus, it may be useful to generate the Legendre product polynomial from its indices, but then to convert it to a standard polynomial form.

The representation of arbitrary multivariate polynomials can be complicated. In this library, we have chosen a representation involving the spatial dimension M, and three pieces of data, O, C and E.

• O is the number of terms in the polynomial.
• C() is a real vector of length O, containing the coefficients of each term.
• E() is an integer vector of length O, which defines the index (the exponents of X(1) through X(M)) of each term.

The exponent indexing is done in a natural way, suggested by the following indexing for the case M = 2:

```        1: x^0 y^0
2: x^0 y^1
3: x^1 y^0
4: x^0 y^2
5: x^1 y^1
6; x^2 y^0
7: x^0 y^3
8: x^1 y^2
9: x^2 y^1
10: x^3 y^0
...
```

### Languages:

legendre_product_polynomial is available in a C version and a C++ version and a Fortran90 version and a MATLAB version and an Octave version and a Python version.

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