1. The problem statement, all variables and given/known data Dear all, i am figuring the taylor expansion for magnetic field in cylindrical coordinate (r,theta,z). The taylor expansion is simple, however if i want to express the magnetic field in just z direction, i don't know to start. It is because i seen some books written: Bz = Bo + B' z + 1/2B'' z^2 - 1/4 B'' r^2 +.... , where B' = partial Bz respect to z B'' = second partial Bz respect to z I couldn't figure out why the '-1/4 B'' r^2' term is here. 2. Relevant equations 3. The attempt at a solution Sorry that i could not how to start, so i just can show my work on the the taylor series of Bz = Bo + B' z + B''z^2/2! +B''' z^3/3! +.... And i know that if the magnetic field is cylindrical , the curl and div of B is zero, and i get the relationship that 1/r partial (rB_r) respect to r + 1/r partial (B_theta) respect to theta + partial (B_z) respect to z = 0 --- (1) (from div B=0) AND 1/r partial (B_z) respect to theta = partial (B_theta) respect to z --- (2) (from curl B= 0) AND partial (B_r) respect to z = partial (B_z) respect to r --- (3) (from curl B= 0) AND partial (rB_theta) respect to r = partial (B_r) respect to theta --- (4) (from curl B= 0) Can anyone help me and give me some ideas? Thank you for your help and attention.