無限級数の紹介

An infinite series is the sum of an infinite sequence of terms, where the addition continues forever. It is defined by the limit of its partial sums.

Imagine a water tank with a smart valve programmed to release exactly half of the remaining water every minute.

In the next minute, half of what’s left drains out—that’s one-quarter. Then one-eighth, then one-sixteenth. This pattern continues forever, with each amount being half of the one before.

At first, it feels like a paradox. If you are always removing something, it seems the tank should never truly be empty.

There’s always a tiny, microscopic drop remaining. But as time goes on, that remaining amount becomes so small it effectively disappears.

To track this, mathematicians use partial sums, a running total of all the water that has left the tank.

If these running totals approach a fixed number, the series converges. In this case, the sum approaches the total volume of the tank.

However, some series don't settle on a finite value. Compounded wealth, for example, multiplies itself by a fixed interest rate every interval, causing the total to grow endlessly in a divergent series.