@neauoire it reminds me a little technique i had found somewhere (but i don't remember where) when i was at school, to multiply 2 polynomials. You put the coefficients on lines and columns of a table, you multiply them in each cell, and you add the diagonals to obtain the result. It looks like we can do it with your technique too, except you have to invent a way to deal with negative numbers ☺️