A pure and an applied mathematician

A pure and applied mathematician are asked to calculate 2 * 2.

The applied mathematician’s solution:

We have,

2 * 2 = 2 *1/ (1-1/2).

The second factor on the right hand side has a geometric series expansion

1/(1-1/2) = 1 + 1/2 +1/4 + 1/8 + ….

Cutting off the series after the second term yields the approximate solution

2 * 2 = 2 *(1 +1/2) = 3.

The pure mathematician’s solution:

We have,

2 * 2 = (-2) *1/(1-3/2).

The second factor on the right hand side has a geometric series expansion

1/(1-3/2) = 1 + 3/2 +9/4 + 27/8 + …,

which diverges.

Hence, the solution to 2 * 2 does not exist.

Collected by Xongkhya Das.