A surprise

Yesterday evening, taking a passing interest in the sinc function, I was stuck by the expansion snapped above.

For some reason, it seemed bizarre that someone should have formulated and proved such a thing so long ago, more or less at the dawn of modern mathematics. At a time when much scholarly interchange was conducted by hand written letter, rather than in person, by telephone or by email. A discovery quite possibly made in St. Petersburg, where the Swiss-born Euler spent most of his professional life.

Reference 1: https://en.wikipedia.org/wiki/Sinc_function.

Reference 2: https://en.wikipedia.org/wiki/Leonhard_Euler.