The equation $\operatorname{sen} x = \frac{1}{3}$ has infinitely many solutions, but none of them is a notable arc. How to write the solution? We need a name for “the arc whose sine is $\frac{1}{3}$”. The problem is that there are infinitely many arcs like this; so that the name designates a number, we choose an interval where the sine is not repeated. This choice defines the inverse circular functions. The story The notation $\operatorname{arc\,sen} x$ — “the arc whose sine is $x$” — was used by Daniel Bernoulli in 1729 and by the Austrian mathematician Joseph Scherffer in 1772, and gained traction with Lagrange and other authors from the end of the 18th century. The notation $\operatorname{sen}^{-1} x$, common in calculators, was proposed by the English astronomer John Herschel in 1813, by analogy with the inverse of a function; it has the defect of being confused with $\frac{1}{\operatorname{sen} x}$, which is why in this book we use $\operatorname{arc\,sen}$.