Why do I get an erroneous complex resultwhen using SOLVE on an equation containing a symbolic variable called "I"?

I use the SOLVE function and I get a result that has imaginary components. My equation includes a symbolic variable, I, which has been interpreted as the imaginary number i (the square root of -1). This happens despite the SYMVAR function listing I as a symbolic variable.

For instance, create the following equation and list the symbols involved:

eqn = 'x = I^2';

symvar(eqn)

 ans = 

     'I'

     'x'

However, when SOLVE is used, "I" is not treated as a symbol, but instead is treated as "sqrt(-1)":

solve(eqn,'x')

 ans =

 -1

Answer 1

[MathWorks Support Team]

The SYMVAR command is a MATLAB function which does not interface with MuPAD or the symbolic toolbox when interpreting a string. Using the following syntax would give the correct symbolic variables:

symvar(sym(eqn))

Currently, when using strings with SOLVE and DSOLVE, the names inside are the MuPAD names. So "I" is the MuPAD "I" which is sqrt(-1).

The workarounds would be:

1) use another name besides I or

2) use sym objects:

syms x I

solve(x^2-I,x)