Note: All the series on this page are convergent!!!
The series
1 - 1/2 + 1/3 - 1/4 + 1/5 - 1/6 + ......
is a convergent alternating series. Let S be the sum.
Then S = (1 - 1/2) + (1/3 - 1/4) + (1/5 - 1/6) + ... is positive. .......... (1)
On the other hand,
S = 1 - 1/2 - 1/4 + 1/3 - 1/6 - 1/8 + 1/5 - 1/10 - 1/12 + 1/7 - ......
= (1 - 1/2) - 1/4 + (1/3 - 1/6) - 1/8 + (1/5 - 1/10) - 1/12 + ...
= 1/2 - 1/4 + 1/6 - 1/8 + 1/10 - 1/12 + .....
= (1/2) [1 - 1/2 + 1/3 - 1/4 + 1/5 - 1/6 + ...... ]
= S/2 .
Hence, S = 0. But this contradicts (1).