Symbolic Dot Product Of Symbolic 3D Vectors

Question

Adam Hartshorne 2017-3-15

回答： Steven Lord 2017-3-15

I have three 3d symbolic vectors, du, dv, dvv, which I initialize as

duu = sym('duu', [3,1]) 
...

I then calculate

N = cross(du,dv) / norm(cross(du,dv))

which gives me a resulting 3,1 vector as one would expect.

Now I want to do the symbolic dot product of (N, duu), however if I run

E = dot(N,duu)

the answer contains a load conj.

How to do I enforce that the result for E is a vector of real symbolic variables, not complex ones?

Answer 1

Steven Lord 2017-3-15

Tell Symbolic Math Toolbox to assume that your symbolic objects are real either when you construct them:

duu2 = sym('duu2', [3,1], 'real')
dot(duu2, duu2)

or after they are constructed using assume or assumeAlso.

duu3 = sym('duu3', [3,1])
assume(duu3, 'real')
dot(duu3, duu3)

See this documentation page for more information about setting assumptions on symbolic objects.