Chapter 8: Infinite Sequences and Series
Section 8.3: Convergence Tests
Example 8.3.6
Determine if the series ∑n=2∞−1n lnnn diverges, converges absolutely, or converges conditionally.
If it converges conditionally, determine if it also converge absolutely.
Solution
Mathematical Solution
The series is alternating, suggesting the Leibniz test. For this, lnn/n must eventually become a monotone decreasing sequence with limit zero. Consequently, the following calculations are executed.
limn→∞lnn/n = 0 and ddxlnx/x=2−lnx2 x3/2<0 for lnx>2, or x>e2≐7.4
The conditions of the Leibniz test apply, and by that test, the series converges conditionally.
To test for absolute convergence, apply the Integral test: ∫8∞lnxx ⅆx = ∞.
By this test, the series of absolute values does not converge, so the series is not absolutely convergent.
Maple Solution
Figure 8.3.6(a) contains a graph of the function fx=lnx/x (in red) and of its derivative (in green).
On the basis of this graph, it may be conjectured that f is monotone decreasing and bounded below by zero, provided x≥8. (The derivative appears to be negative for x>8.)
Consequently, the Integral test may be tried as a test for absolute convergence, provided the integration starts from, say, x=8.
To this end:
Calculus palette: Definite integral template Context Panel: Evaluate and Display Inline
∫8∞lnxx ⅆx = ∞
module() local F,p,N; F:=ln(x)/sqrt(x); N:=15; p:=plot([F,diff(F,x)],x=1..N,color=[red,green],view=[0..N,default],tickmarks=[N,default],labels=[x,y]); print(p); end module:
Figure 8.3.6(a) Graph of fx (red) and f′x (green)
Since the integral diverges, the series does not converge absolutely.
To establish the conditional convergence by the Leibniz test, implement the following two calculations.
Calculus palette: Limit operator
Context Panel: Evaluate and Display Inline
limn→∞lnnn = 0
Calculus palette: Differentiation operator
ⅆⅆ x lnxx = 1x3/2−12⁢ln⁡xx3/2
A modicum of algebra puts the derivative into the form 2−lnx2 x3/2, from which it can be seen that the derivative is negative for 2−lnx<0, or x>e2≐7.4. (This is consistent with Figure 8.3.6(a).) Since the criteria of the test are satisfied, it follows that the series converges conditionally.
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