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EXAMPLE 2Using the Ratio Test

Use the Ratio Test to determine whether the series k=1k!kk converges or diverges.

Solutionk=1k!kk is a series of nonzero terms. Since an+1=(n+1)!(n+1)n+1 and an=n!nn, the absolute value of their ratio is |an+1an|=(n+1)!(n+1)n+1n!nn=(n+1)!(n+1)n+1nnn!=nn(n+1)n=(nn+1)n=(11+1n)n=1(1+1n)n

So, lim

Since \dfrac{1}{e}<1, the series converges.

The number e expressed as a limit is discussed in Section 3.3, pp. 227-228.