To find the convergence of the series `sum_(n=2)^oo (ln(n))/n^p` where `pgt0` (positive values of `p` ), we may apply integral test.
Integral test is applicable if f is positive, continuous, and decreasing function on an interval and let `a_n=f(x).` Then the infinite series `sum_(n=k)^oo a_n` converges if and only if the improper integral `int_k^oo f(x) dx` converges to a real number. If the integral diverges then the series also diverges.
For the infinte series series `sum_(n=2)^oo (ln(n))/n^p ` , we have:
`a_n =(ln(n))/n^p`
Then, `f(x) =(ln(x))/x^p`
The `f(x)` satisfies the conditions for integral test when `pgt0` . We set-up the improper integral as:
`int_2^oo (ln(x))/x^pdx`
Apply integration by parts: `int u dv = uv - int v du.`
Let: `u=ln(x)` then `du = 1/xdx`
`dv = 1/x^p dx`
Then , `v = int dv`
`=int 1/x^p dx `
`= int x^(-p) dx`
`= x^(-p+1)/(-p+1)`
The indefinite integral will be:
`int (ln(x))/x^pdx = ln(x)x^(-p+1)/(-p+1)- intx^(-p+1)/(-p+1) *1/x dx`
`= ln(x)x^(-p+1)/(-p+1)-1/(-p+1) int (x^(-p)x)/x dx`
`= ln(x)x^(-p+1)/(-p+1)-1/(-p+1) intx^(-p) dx `
`= ln(x)x^(-p+1)/(-p+1)-1/(-p+1) *x^(-p+1)/(-p+1)`
` =(ln(x)x^(-p+1))/(-p+1)-x^(-p+1)/(-p+1)^2`
`=(ln(x)x^(-p+1))/(-p+1)*(-p+1)/(-p+1)-x^(-p+1)/(-p+1)^2`
`=(ln(x)x^(-p+1)(-p+1))/(-p+1)^2-x^(-p+1)/(-p+1)^2`
`=(ln(x)x^(-p+1)(-p+1)-x^(-p+1))/(-p+1)^2|_2^oo`
The definite integral will only be finite if `1-p<0 or pgt1` .
Thus, the series `sum_(n=2)^oo(ln(n))/n^p` converges when `pgt1` .
See eNotes Ad-Free
Start your 48-hour free trial to get access to more than 30,000 additional guides and more than 350,000 Homework Help questions answered by our experts.
Already a member? Log in here.