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Old November 5th, 2009, 10:43 PM
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Default Period : 2

Find the period for the function f in R:
f(x+1)f(x)+f(x+1)+1=f(x)
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Old November 6th, 2009, 03:27 AM
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We have f(x+1)=\frac{f(x)-1}{f(x)+1}.

Replace x with x+1:

f(x+2)=\frac{f(x+1)-1}{f(x+1)+1}=\frac{\displaystyle\frac{f(x)-1}{f(x)+1}-1}{\displaystyle\frac{f(x)-1}{f(x)+1}+1}=-\frac{1}{f(x)}

Replace x with x+2:

f(x+4)=-\frac{1}{f(x+2)}=-\frac{1}{\displaystyle -\frac{1}{f(x)}}=f(x).

Then the period is 4.
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