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)
SERIESSUM(x;n;m;係数) = 係数1*x^n + 係数2*x^(n+m) + 係数3*x^(n+2m) +...+ 係数i*x^(n+(i-1)m)