LU factorization for a tridigonal matrix in VBA

Option Base 1
Sub Tridiagonal_solver()
A_Mat = Range("A1:E5")
coeff = Range("H1:H5")
Dim n As Integer
n = UBound(A_Mat, 1)
Dim l_mat() As Double, u_mat() As Double, alpha As Double, beta As Double, y_array() As Double
ReDim y_array(n) As Double
ReDim l_mat(n, n) As Double
ReDim u_mat(n, n) As Double
For i = 1 To n
For j = i To n
l_mat(i, j) = 0
u_mat(i, j) = 0
Next j, i

For i = 1 To n
If (i = 1) Then
u_mat(i, i) = A_Mat(i, i)
End If
l_mat(i, i) = 1

If (i > 1) Then
u_mat(i, i - 1) = A_Mat(i, i - 1)
l_mat(i, i - 1) = A_Mat(i, i - 1) / u_mat(i - 1, i - 1)
u_mat(i, i) = A_Mat(i, i) - l_mat(i, i - 1) * A_Mat(i - 1, i)
End If
Next i
For i = 1 To n
If (i = 1) Then
y_array(i) = coeff(i)
End If

Next i
Range("A8:E12") = l_mat
Range("H8:L12") = u_mat

End Sub