# I want my plotting function to treat numbers as numbers, not variables

I'm creating a plot, like this:

```
density_plot(f(x, y), (x, 0, 1), (y, 0, 1))
```

The function *f* looks basically like this:

```
def f(x, y):
u = vector([x, y, 1 - x - y])
return u.norm(2)
```

That works. Now I introduce a vector v like this:

```
def f(x, y):
u = vector([x, y, 1 - x - y])
v = vector([5, 4, 2])
return (u - v).norm(2)
```

That works. But if v is the solution of an equation, like this:

```
v = M.right_kernel().basis()[0]
```

Now Sage can't handle the subtraction u - v:

TypeError: unsupported operand parent(s) for '-': 'Vector space of dimension 3 over Symbolic Ring' and 'Vector space of degree 3 and dimension 1 over Real Field with 53 bits of precision Basis matrix:[ 1.00000000000000 0.571428571428571 1.57142857142857]'

So okay, I get that basically what's happening is that u consists of symbolic polynomials in x and y, but v consists of actual numbers, or something like that. But why is it not a problem when v is defined explicitly by `vector([5, 5, 5])`

or whatever? What is it about the object returned by `.right_kernel().basis()[0]`

(which in all other respects behaves like a vector) that's so incompatible in this context?

How can I solve the matrix equation Mx = 0 in such a way that the vector I get can be subtracted from u? Note that M is singular in my case, with nullity 1, so I need to be able to get an arbitrary vector out of its nullspace.

Can you tell us exactly how you define

`M`

? I tried a few experiments with this and couldn't reproduce your error.This matrix seems to reproduce it:

Thanks for reporting, thanks @kcrisman for the ticket, it is now trac ticket 21777

Oops, sorry for not reporting the ticket here - I must have forgotten.