The first English professor of mathematics
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You can search for a mathematician, find who was their advisor, and their advisor, and so on.
Lagrange also advised Fourier, Schwartz also advised Grothendieck, Borel also advised Lebesgue etc... Lagrange was advised by Euler, and Laplace by D'Alembert
So another path is Euler -> Lagrange -> Fourier -> Navier
another one: D'Alembert -> Laplace -> Poisson -> Dirichlet -> Lipschitz -> Felix Klein -> Lindemann -> Hilbert etc etc
Galileo Galilei, Torricelli -> Vincenzo Viviani -> Isaac Barrow -> Isaac Newton
Or did they simply write in out by hand on paper like on is usually taught in elementary school?
These were often evaluated as Newton approximation, Taylor expansion or direct series if it converged fast enough.
Additionally the first accurate multiplication algorithm faster than long multiplication has only been devised in XX century - Karatsuba's.
Abacus is just a mechanical tool that represents long addition and multiplication.