A class of orthogonal polynomials Hn(x) that are solutions to the following ordinary differential equation:

``` 2
d y      dy
--- - 2x -- + ny = 0
2      dx
dx
```

The polynomials are given by the Rodrigues formula:

```                       n
n      2  d        2
H (x) = (-1)  exp(x ) ---exp(-x )
n                      n
dx
```

and satisfy the three-term recurrence relation:

```H   (x) = 2xH (x) - 2nH   (x)
n+1         n         n-1
```

They are also orthogonal over the range (-∞, ∞) with weight exp(-x2):

``` ∞                 2           n
∫  H (x)H (x)exp(-x ) dx = δ  2 n!sqrt(π)
-∞ m    n                  mn
```

The first few polynomials are:

```H (x) = 1
0
H (x) = 2x
1
2
H (x) = 4x  - 2
2
3
H (x) = 8x  - 12x
3
```

These polynomials are also related to the confluent hypergeometric function by the relation:

```          n
2 sqrt(π)                2
H (x) = ----------- M(-n/2; 1/2; x ) -
n        γ(1-n/2)
n+1
2   sqrt(π)                  2
-------------xM((1-n)/2; 3/2; x )
γ(-n/2)
```

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