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 #18
avatar+11912 
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Jun 17, 2014
 #2
avatar+831 
+5
Jun 17, 2014
 #1
avatar+118723 
+5

Let me see.

Let the effective 4 monthly interest rate be r

Now I need to find the effective 3 yearly interest rate equivalent.  

There are 9 four monthly periods in 3 years so  the effective 3 yearly rate is (1+r)9 -1

Consider any perpetuity.

Let the amount at the beginning of a period be A and the period interest rate be i  then the interest earned in any period is Ai.  Now if this is to pay out into perpetuity then the regular period payment must equal the period interest. 

That is  A+Ai-R=A

Ai=R  (I have used A instead of PV)

now I want to look at the 3 year perpetuity.  

Initial investment = €32

after 3 years = €32[ (1+r)9 -1+1] = €32[ (1+r)9 ]  this is where the perpetuity actually begins. so this is A

A= €32 (1+r)9

i = (1+r)-1

$$\begin{array}{rll}
Ai&=&R\\\\
\left[32(1+r)^{9}\right]\left[(1+r)^{9}-1\right]&=&10 \\\\
32(1+r)^{18}-32(1+r)^{9}-10&=&0\\\\
16(1+r)^{18}-16(1+r)^{9}-5&=&0\\\\
\mbox{let } x=(1+r)^{9}&&\\\\
16x^2-16x-5&=&0 \mbox{ where } x > 0\\\\
\mbox{the quadratic formula gives me }x&=&1.25\\\\
(1+r)^{9}&=&1.25\\\\
1+r&=&1.25^{1/9}\\\\
r&=&1.25^{1/9}-1\approx 0.025103648\\\\
\end{array}$$

Okay now lets look at the 4 monthly perpetuity

Ai=R, A=unknown, i=r=etc, R=€1 (is that what you mean by 1 unit?)

$$\begin{array}{rll}
A&=&\frac{R}{i}\\\\
A&=&\frac{1}{1.25^{1/9}-1}\\\\
A&=&39.83484719\\\\
A&=& 39.83
\end{array}$$

 PV=€39.83

I think that is right but I haven't done any checking.

Can you do that reinout?  I might but it is 1:15am here.  You might be a little less tired than I.

Jun 17, 2014
 #2
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Jun 17, 2014

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