Definition:QEI

Definition
The initials of Quod Erat Inveniendum, which is for which was to be found.

These initials were traditionally added after the completion of a calculation (either arithmetic or algebraic).

This is practically never seen nowadays outside translations of historical texts.

If there were ever a need for such an indication, the Halmos symbol $\blacksquare$ would be used instead.

does not bother to distinguish between use of QEI and the better-known QED, and recommends the use of the qed template exclusively instead.

Also known as
Some sources render it as Q.E.I. (that is, with dots).