Tuesday 23 July 2013

proof writing - Proving sequence statement using mathematical induction, $d_n = frac{2}{n!}$



I'm stuck on this homework problem. I must prove the statement using mathematical induction




Given: A sequence $d_1, d_2, d_3, ...$ is defined by letting $d_1 = 2$ and for all integers k $\ge$ 2.
$$
d_k = \frac{d_{k-1}}{k}
$$



Show that for all integers $n \ge 1$ , $$d_n = \frac{2}{n!}$$







Here's my work:



Proof (by mathematical induction). For the given statement, let the property $p(n)$ be the equation:



$$
d_n = \frac{2}{n!}
$$



Show that $P(1)$ is true:
The left hand side of $P(1)$ is $d_n$ , which equals $2$ by definition of the sequence.

The right hand side is:



$$ \frac{2}{(1)!} =2 $$



Show for all integers $k \geq 1$, if $P(k)$ is true, then $p(k+1)$ is true.
Let k be any integer with $k \geq 1$, and suppose $P(k)$ is true. That is, suppose: (This is the inductive hypothesis)



$$ d_{k} = \frac{2}{k!} $$



We must show that $P(K+1)$ is true. That is, we must show that:




$$ d_{k+1} = \frac{2}{(k+1)!} $$



(I thought I was good until here.)



But the left hand side of $P(k+1)$ is:



$$ d_{k+1} = \frac{d_k}{k+1} $$



By inductive hypothesis:




$$ d_{k+1} = \frac{(\frac{2}{2!})}{k+1} $$



$$ d_{k+1} = \frac{2}{2!}\frac{1}{k+1} $$



but that doesn't seem to equal what I needed to prove: $ d_n = \frac{2}{n!}$


Answer



The following is not true $$d_{k+1} = \frac{(\frac{2}{2!})}{k+1}$$ since $d_k=\frac{2}{k!}$ not $\frac{2}{2!}$, you actually have $$d_{k+1} = \frac{(\frac{2}{k!})}{k+1}=\frac{(\frac{2}{k!(k+1)})}{1}=\frac{2}{(k+1)!}$$


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