Which mathematician introduced the letter i for the square root of minus one?

Leonhard Euler introduced i as the symbol for the square root of minus one in the eighteenth century, and the notation stuck because it made an awkward idea easy to manipulate. Before that, such quantities were treated with suspicion, which is why Rene Descartes had labelled them imaginary as a dismissal. Euler took them seriously and used them freely, and his identity, which links e raised to the power of i times pi with minus one, ties together five of the most important constants in a single line. He was extraordinarily productive, and much of the notation students meet today is his: the function notation f of x, the symbol e for the base of natural logarithms, capital sigma for summation and the use of pi in its modern sense all trace back to him or were spread by his textbooks. Complex numbers turned out to be indispensable, especially in electrical engineering, signal processing and quantum mechanics.

Electrical engineers write j instead of i, because i is already taken by current.