# 5th May

On this day in 1777, Leonhard Euler was the first to use the symbol

See THIS LINK.

*i*for $\sqrt{-1}$ in the paper*De Formulis Differentialibus Angularibus maxime irrationalibus quas tamen per logarithmos et arcus circulares integrare licet*[Of the most irrational Angular Differential Formulas, which, however, may be integrated by means of logarithms and circular arcs], presented to the St Petersburg Academy:-Quoniam mihi quidem alia adhuc via non patet istud praestandi nisi per imaginaria procedendo, formulam $\sqrt{-1}$ littera i in posterum designabo, ita ut sit i i = -1 ideoque 1/i = -i.[Since there is still no other way clear to me of accomplishing this except by an imaginary procedure, I will denote the formula $\sqrt{-1}$ by the letter

*i*in the future, so that*i i*= -1 and therefore 1/*i*= -*i*.]See THIS LINK.