## VRdRFFTMSq

#### Routine

void VRdRFFTMSq (const double X[], double X2[], int N)

#### Purpose

Calculate the magnitude squared spectrum for the DFT of a real sequence

#### Description

This routine calculates the magnitude squared of the DFT of a real sequence. The output of the DFT (for instance SPdRFFT) is stored in an array with N elements, with the real part preceding the imaginary part. This DFT output is the input to this routine.

The input values to this routine are ar(.), the real part of the DFT, and ai(.), the imaginary part of the DFT. For N even, the values are stored as follows.

```    ar(0)     <->  X
ar(1)     <->  X
...           ...
ar(N/2-1) <->  X[N/2-1]
ar(N/2)   <->  X[N/2]
ai(1)     <->  X[N/2+1]
ai(2)     <->  X[N/2+2]
...           ...
ai(N/2-1) <->  X[N-1]
```
The values ai(0) and ai(N/2) are known, a priori, to be zero. The output of this routine is a vector of N/2+1 values calculated as follows.
```    X2[k] = ar(k)^2 + ai(k)^2   0 <= k <= N/2.
```

This routine also works for N odd, in which case there are (N+1)/2 output values.

#### Parameters

-> const double X[]
Input array (N elements)
<- double X2[]
Output array (N/2+1 (N even) or (N+1)/2 (N odd) elements). The output array can be the same as the input array.
-> int N
Number of elements in the input array (may be zero)

#### Author / revision

P. Kabal / Revision 1.3 2003/05/09

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