Assumptions

class sage.symbolic.assumptions.GenericDeclaration(var, assumption)

Bases: sage.structure.sage_object.SageObject

This class represents generic assumptions, such as a variable being an integer or a function being increasing. It passes such information to maxima’s declare (wrapped in a context so it is able to forget).

INPUT:

  • var – the variable about which assumptions are being made
  • assumption – a string containing a Maxima feature, either user defined or in the list given by maxima('features')

EXAMPLES:

sage: from sage.symbolic.assumptions import GenericDeclaration
sage: decl = GenericDeclaration(x, 'integer')
sage: decl.assume()
sage: sin(x*pi)
sin(pi*x)
sage: sin(x*pi).simplify()
0
sage: decl.forget()
sage: sin(x*pi)
sin(pi*x)
sage: sin(x*pi).simplify()
sin(pi*x)

Here is the list of acceptable features:

sage: maxima('features')
[integer,noninteger,even,odd,rational,irrational,real,imaginary,complex,analytic,increasing,decreasing,oddfun,evenfun,posfun,constant,commutative,lassociative,rassociative,symmetric,antisymmetric,integervalued]
assume()

Make this assumption.

TEST:

sage: from sage.symbolic.assumptions import GenericDeclaration
sage: decl = GenericDeclaration(x, 'even')
sage: decl.assume()
sage: cos(x*pi).simplify()
1
sage: decl2 = GenericDeclaration(x, 'odd')
sage: decl2.assume()
Traceback (most recent call last):
...
ValueError: Assumption is inconsistent
sage: decl.forget()
contradicts(soln)

Return True if this assumption is violated by the given variable assignment(s).

INPUT:

  • soln – Either a dictionary with variables as keys or a symbolic relation with a variable on the left hand side.

EXAMPLES:

sage: from sage.symbolic.assumptions import GenericDeclaration
sage: GenericDeclaration(x, 'integer').contradicts(x==4)
False
sage: GenericDeclaration(x, 'integer').contradicts(x==4.0)
False
sage: GenericDeclaration(x, 'integer').contradicts(x==4.5)
True
sage: GenericDeclaration(x, 'integer').contradicts(x==sqrt(17))
True
sage: GenericDeclaration(x, 'noninteger').contradicts(x==sqrt(17))
False
sage: GenericDeclaration(x, 'noninteger').contradicts(x==17)
True
sage: GenericDeclaration(x, 'even').contradicts(x==3)
True
sage: GenericDeclaration(x, 'complex').contradicts(x==3)
False
sage: GenericDeclaration(x, 'imaginary').contradicts(x==3)
True
sage: GenericDeclaration(x, 'imaginary').contradicts(x==I)
False

sage: var('y,z')
(y, z)
sage: GenericDeclaration(x, 'imaginary').contradicts(x==y+z)
False

sage: GenericDeclaration(x, 'rational').contradicts(y==pi)
False
sage: GenericDeclaration(x, 'rational').contradicts(x==pi)
True
sage: GenericDeclaration(x, 'irrational').contradicts(x!=pi)
False
sage: GenericDeclaration(x, 'rational').contradicts({x: pi, y: pi})
True
sage: GenericDeclaration(x, 'rational').contradicts({z: pi, y: pi})
False
forget()

Forget this assumption.

TEST:

sage: from sage.symbolic.assumptions import GenericDeclaration
sage: decl = GenericDeclaration(x, 'odd')
sage: decl.assume()
sage: cos(pi*x)
cos(pi*x)
sage: cos(pi*x).simplify()
-1
sage: decl.forget()
sage: cos(x*pi).simplify()
cos(pi*x)
has(arg)

Check if this assumption contains the argument arg.

EXAMPLES:

sage: from sage.symbolic.assumptions import GenericDeclaration as GDecl
sage: var('y')
y
sage: d = GDecl(x, 'integer')
sage: d.has(x)
True
sage: d.has(y)
False
sage.symbolic.assumptions.assume(*args)

Make the given assumptions.

INPUT:

  • *args – assumptions

EXAMPLES:

Assumptions are typically used to ensure certain relations are evaluated as true that are not true in general.

Here, we verify that for \(x>0\), \(\sqrt{x^2}=x\):

sage: assume(x > 0)
sage: bool(sqrt(x^2) == x)
True

This will be assumed in the current Sage session until forgotten:

sage: forget()
sage: bool(sqrt(x^2) == x)
False

Another major use case is in taking certain integrals and limits where the answers may depend on some sign condition:

sage: var('x, n')
(x, n)
sage: assume(n+1>0)
sage: integral(x^n,x)
x^(n + 1)/(n + 1)
sage: forget()
sage: var('q, a, k')
(q, a, k)
sage: assume(q > 1)
sage: sum(a*q^k, k, 0, oo)
Traceback (most recent call last):
...
ValueError: Sum is divergent.
sage: forget()
sage: assume(abs(q) < 1)
sage: sum(a*q^k, k, 0, oo)
-a/(q - 1)
sage: forget()

An integer constraint:

sage: var('n, P, r, r2')
(n, P, r, r2)
sage: assume(n, 'integer')
sage: c = P*e^(r*n)
sage: d = P*(1+r2)^n
sage: solve(c==d,r2)
[r2 == e^r - 1]

Simplifying certain well-known identities works as well:

sage: sin(n*pi)
sin(pi*n)
sage: sin(n*pi).simplify()
0
sage: forget()
sage: sin(n*pi).simplify()
sin(pi*n)

If you make inconsistent or meaningless assumptions, Sage will let you know:

sage: assume(x<0)
sage: assume(x>0)
Traceback (most recent call last):
...
ValueError: Assumption is inconsistent
sage: assume(x<1)
Traceback (most recent call last):
...
ValueError: Assumption is redundant
sage: assumptions()
[x < 0]
sage: forget()
sage: assume(x,'even')
sage: assume(x,'odd')
Traceback (most recent call last):
...
ValueError: Assumption is inconsistent
sage: forget()

You can also use assumptions to evaluate simple truth values:

sage: x, y, z = var('x, y, z')
sage: assume(x>=y,y>=z,z>=x)
sage: bool(x==z)
True
sage: bool(z<x)
False
sage: bool(z>y)
False
sage: bool(y==z)
True
sage: forget()
sage: assume(x>=1,x<=1)
sage: bool(x==1)
True
sage: bool(x>1)
False
sage: forget()

TESTS:

Test that you can do two non-relational declarations at once (fixing trac ticket #7084):

sage: var('m,n')
(m, n)
sage: assume(n, 'integer'); assume(m, 'integer')
sage: sin(n*pi).simplify()
0
sage: sin(m*pi).simplify()
0
sage: forget()
sage: sin(n*pi).simplify()
sin(pi*n)
sage: sin(m*pi).simplify()
sin(pi*m)
sage.symbolic.assumptions.assumptions(*args)

List all current symbolic assumptions.

INPUT:

  • args – list of variables which can be empty.

OUTPUT:

  • list of assumptions on variables. If args is empty it returns all assumptions

EXAMPLES:

sage: var('x, y, z, w')
(x, y, z, w)
sage: forget()
sage: assume(x^2+y^2 > 0)
sage: assumptions()
[x^2 + y^2 > 0]
sage: forget(x^2+y^2 > 0)
sage: assumptions()
[]
sage: assume(x > y)
sage: assume(z > w)
sage: list(sorted(assumptions(), lambda x,y:cmp(str(x),str(y))))
[x > y, z > w]
sage: forget()
sage: assumptions()
[]

It is also possible to query for assumptions on a variable independently:

sage: x, y, z = var('x y z')
sage: assume(x, 'integer')
sage: assume(y > 0)
sage: assume(y**2 + z**2 == 1)
sage: assume(x < 0)
sage: assumptions()
[x is integer, y > 0, y^2 + z^2 == 1, x < 0]
sage: assumptions(x)
[x is integer, x < 0]
sage: assumptions(x, y)
[x is integer, x < 0, y > 0, y^2 + z^2 == 1]
sage: assumptions(z)
[y^2 + z^2 == 1]
sage.symbolic.assumptions.forget(*args)

Forget the given assumption, or call with no arguments to forget all assumptions.

Here an assumption is some sort of symbolic constraint.

INPUT:

  • *args – assumptions (default: forget all assumptions)

EXAMPLES:

We define and forget multiple assumptions:

sage: var('x,y,z')
(x, y, z)
sage: assume(x>0, y>0, z == 1, y>0)
sage: list(sorted(assumptions(), lambda x,y:cmp(str(x),str(y))))
[x > 0, y > 0, z == 1]
sage: forget(x>0, z==1)
sage: assumptions()
[y > 0]
sage: assume(y, 'even', z, 'complex')
sage: assumptions()
[y > 0, y is even, z is complex]
sage: cos(y*pi).simplify()
1
sage: forget(y,'even')
sage: cos(y*pi).simplify()
cos(pi*y)
sage: assumptions()
[y > 0, z is complex]
sage: forget()
sage: assumptions()
[]
sage.symbolic.assumptions.preprocess_assumptions(args)

Turn a list of the form (var1, var2, ..., 'property') into a sequence of declarations (var1 is property), (var2 is property), ...

EXAMPLES:

sage: from sage.symbolic.assumptions import preprocess_assumptions
sage: preprocess_assumptions([x, 'integer', x > 4])
[x is integer, x > 4]
sage: var('x, y')
(x, y)
sage: preprocess_assumptions([x, y, 'integer', x > 4, y, 'even'])
[x is integer, y is integer, x > 4, y is even]

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