SERIESSUM(x; n; m; coefficients) = coefficient_1*x^n + coefficient_2*x^(n+m) + coefficient_3*x^(n+2*m) + ... + coefficient_i*x^(n+(i-1)*m)
SOMME.SERIES(x;n;m;coefficients) = coefficient_1*x^n + coefficient_2*x^(n+m) + coefficient_3*x^(n+2m) +...+ coefficient_i*x^(n+(i-1)m)