Modern Arithmetic begins with Leonardi Fibonacci’s treatise “de Abaco,”
written in 1202 and re-written in 1228. It is modern rather in the range
of its problems and the methods of attack than in mere methods of
calculation, which are of its period. Its sole interest as regards the
present work is that Leonardi makes use of the digital signs described
in Record’s treatise on _The arte of nombrynge by the hand_ in mental
arithmetic, calling it “modus Indorum.” Leonardo also introduces the
method of proof by “casting out the nines.”