Error #Value!, vba code trinomial model, Option Put American

I am trying to calculate the PUT PRICE but I am getting error Value. This is the code :

[CODE
Function Mon_AmericanPut_Trinomial(S, X, T, rf, sigma, n)
delta_t = T / n
up = Exp(sigma * Sqr(3 * delta_t))
down = 1 / up
r = Exp(rf * delta_t)
Pu = (r - down) / (up - down)
Pd = (up - r) / (up - down)
Pm = 1 - Pu - Pd
Dim Prix_t_plus_1() As Double
Dim Prix_t() As Double
ReDim Prix_t_plus_1(n)
For State = 0 To n
Prix_t_plus_1(State) = Application.Max(X - S * _
up ^ State * down ^ (n - State), 0)
Next State
For Index = n - 1 To 0 Step -1
ReDim Prix_t(Index)
For State = 0 To Index
Prix_t(State) = Application.Max(X - S * up ^ State * down ^ (Index - State), _
(r) ^ (-1) * ((Pd) * Prix_t_plus_1(State) + (Pm) * Prix_t_plus_1(State + 1) + (Pu) * Prix_t_plus_1(State + 2)))
Next State
ReDim Prix_t_plus_1(Index)
For State = 0 To Index
Prix_t_plus_1(State) = Prix_t(State)
Next State
Next Index
Mon_AmericanPut_Trinomial = Prix_t(0)
End Function
[/CODE]

1. THERE'S NO NEED TO SHOUT IN YOUR TITLE

2. Please use CODE tags to format your macros. The [#] icon inserts them and paste the code between them. I fixed your macro this time

3. It's hard to test your macro without some input data that causes the error and the expected result. Please add this missing information

Thank you, I did not mean to shout in the title sorry about that.
s= 30
X= 30
T= 1
Sigma= 25%
n = 30
rf=4%

Is OK

?? rf = %

I corrected it

Can't help with math, but in

Code:

---

For State = 0 To Index

---

I think the (state + 2) caused it to run off the edge

I added a - 2 to the For and it at least runs and doesn't give an error

Attachment 30702

Code:

---

Option Explicit

Function Mon_AmericanPut_Trinomial(S As Double, X As Double, T As Double, rf As Double, sigma As Double, n As Long) As Double
Dim Prix_t_plus_1() As Double, delta_t As Double
Dim up As Double, down As Double, r As Double
Dim Prix_t() As Double
Dim Pu As Double, Pd As Double, Pm As Double
Dim State As Long, Index As Long
ReDim Prix_t_plus_1(n)
delta_t = T / n
up = Exp(sigma * Sqr(3 * delta_t))
down = 1 / up
r = Exp(rf * delta_t)
Pu = (r - down) / (up - down)
Pd = (up - r) / (up - down)
Pm = 1 - Pu - Pd
For State = 0 To n
    Prix_t_plus_1(State) = Application.Max(X - S * _
    up ^ State * down ^ (n - State), 0)
Next State
For Index = n - 1 To 0 Step -1
    ReDim Prix_t(Index)
    For State = 0 To Index - 2      '  <<<<<<<<<<<<<<<<<<<<<
          Prix_t(State) = Application.Max(X - S * up ^ State * down ^ (Index - State), _
          (r) ^ (-1) * ((Pd) * Prix_t_plus_1(State) + (Pm) * Prix_t_plus_1(State + 1) + (Pu) * Prix_t_plus_1(State + 2)))
    Next State
    ReDim Prix_t_plus_1(Index)
    For State = 0 To Index
        Prix_t_plus_1(State) = Prix_t(State)
    Next State
Next Index
Mon_AmericanPut_Trinomial = Prix_t(0)
End Function

---