Method on @sym: x = assume (x, cond, cond2, …) ¶

Method on @sym: x = assume (x, 'clear') ¶

Method on @sym: [x, y] = assume ([x y], …) ¶

Method on @sym: assume (x, cond, cond2, …) ¶

Method on @sym: assume (x, 'clear') ¶

Method on @sym: assume ([x y], …) ¶

New assumptions on a symbolic variable (replace old if any).

This function has two different behaviours depending on whether it has an output argument or not. The first form is simpler; it returns a new sym with assumptions given by cond, for example:

syms x
x1 = x;
x = assume(x, 'positive');
assumptions(x)
  ⇒ ans =
    {
      [1,1] = x: positive
    }
assumptions(x1)  % empty, x1 still has the original x
  ⇒ ans = {}(0x0)

Another example to help clarify:

x1 = sym('x', 'positive')
  ⇒ x1 = (sym) x
x2 = assume(x1, 'negative')
  ⇒ x2 = (sym) x
assumptions(x1)
  ⇒ ans =
    {
      [1,1] = x: positive
    }
assumptions(x2)
  ⇒ ans =
    {
      [1,1] = x: negative
    }

The second form—with no output argument—is different; it attempts to find all instances of symbols with the same name as x and replace them with the new version (with cond assumptions). For example:

syms x
x1 = x;
f = sin(x);
assume(x, 'positive');
assumptions(x)
  ⇒ ans =
    {
      [1,1] = x: positive
    }
assumptions(x1)
  ⇒ ans =
    {
      [1,1] = x: positive
    }
assumptions(f)
  ⇒ ans =
    {
      [1,1] = x: positive
    }

To clear assumptions on a variable use assume(x, 'clear'), for example:

syms x positive
f = sin (x);
assume (x, 'clear')
isempty (assumptions (f))
  ⇒ ans = 1

Warning: the second form operates on the caller’s workspace via evalin/assignin. So if you call this from other functions, it will operate in your function’s workspace (and not the base workspace). This behaviour is for compatibility with other symbolic toolboxes.

FIXME: idea of rewriting all sym vars is a bit of a hack, not well tested (for example, with global vars.)

See also: @@sym/assumeAlso, assume, assumptions, sym, syms.

Package: symbolic