Rers45 = 1/sum(1./[R4 R5])
I3 = ((U0b - U0a)*(R3 + Rers45) + (U0c + U0b)*(R1 + R6))/((R3 + Rers45 + R2)*(R1 + R6) + R2*(R3+Rers45))
%I3 = ((-U0a)*(R3 + Rers45) + (U0c + U0b)*(R1 + R6))/((R3 + Rers45 + R2)*(R1 + R6) + R2*(R3+Rers45))
I1 = (I3*(R3 + Rers45 + R2) - U0c - U0b) / (R3 + Rers45)
I1_check = (U0b - U0a - R2*I3)/(R1 + R6)