I am using the Math.NET Numerics library to have access to their Matrix and Vector classes, as well as access to some linear algebra stuff.

I’m porting a simple single layer neural network from C# to F# in an attempt to learn some F#. I need to average some matrices but the class does not seem to play well with the Seq.average and Seq.sum functions. I cannot seem to find anyone capable of telling me how to sum without using Seq.sum.

```
let ComputeDerivative (data : Matrix<float>) (theta : Matrix<float>) : Matrix<float> =
let row : Vector<float> = data.Row(0)
let weights : Vector<float> = theta.Column(0)
let range : List<int> = [2 .. row.Count]
let sum =
range
|> Seq.map (fun index -> row.[index] - (weights.[0] * row.[index - 1]) + (weights.[1] * row.[index - 2]) + weights.[2])
|> Seq.mapi (fun index errorSignal -> Matrix<float>.Build.Dense(3, 1, [| -2.0 * errorSignal * row.[index - 1]; -2.0 * errorSignal * row.[index - 2]; -2.0 * errorSignal;|]))
|> Seq.sum
let average = sum / (float range.Length)
average
```

The issue is that Seq.sum will not work because, “Matrix does not support the operator ‘get_Zero’” and Seq.average will not work because Matrix needs to be divided by a float, not an integer.

How can I sum this?

The C# code I’m porting is:

```
private static Matrix<double> ComputeDerivative(in Matrix<double> data, in Matrix<double> theta) {
int length = Math.Max(data.ColumnCount, data.RowCount);
Matrix<double> total = Matrix<double>.Build.Dense(3, 1, 0);
double[] predictedDJI = new double[length],
observedDJI = new double[length];
for (int index = 2; index < length; index++) {
predictedDJI[index] = (theta[0, 0] * data[0, index - 1]) + (theta[1, 0] * data[0, index - 2]) + theta[2, 0];
observedDJI[index] = data[0, index];
double errorSignal = observedDJI[index] - predictedDJI[index];
Matrix<double> dcdTheta = Matrix<double>.Build.Dense(3, 1, new double[] {
-2 * errorSignal * data[0, index - 1],
-2 * errorSignal * data[0, index - 2],
-2 * errorSignal
});
total += dcdTheta;
}
return total / (length - 2);
}
```

Edit: Posted wrong C# snippit