1. The problem statement, all variables and given/known data I cannot remember if I have already posted this question here... However,if that is the case,let it be said that I want to discuss more about it. Let us define 4 vector by 4 co-ordinates (x1,x2,x3,x4) where (x1,x2,x3) are space components (like x,y,z) and x4 is related to time as x4=ict.Express the following equations in tensor notation. (i)The continuity equation: div J+(del*rho/del t)=0 (ii)The wave equation del2ø-(1/c2) [del2 ø/del t2]=0 (iii)What will be the value of j4 instead of above? 2. Relevant equations 3. The attempt at a solution For the first two part I am getting these: Please check... Let me know if I am correct. (i) (∂/∂x_i)(J_i)+ (∂/∂t)ρ=0 (ii) [∂^2/(∂x_i*∂x_i)]ψ-(1/c^2)[(∂^2/∂t^2)ψ]=0 I have changed phi to psi... Do not mind. How can I get the 3rd part?