Given a symbolic expression like:
sage: var("a b c x y")
(a, b, c, x, y)
sage: a*x^2 + a*y^2 + b*y^2 + c*y^2 + (2*a*y + b*y)*x
How can I manually collect terms together? I want to represent this equation in the form:
$$ Ax^2 + Bxy + Cy^2 $$
Where $A = a, B = 2a + b, C = a + b + c$. Desired output should be:
a*x^2 + (2*a + b)*x*y + (a + b + c)*y^2
And then is there any way to read off these coefficients? Thanks
Does this :
sage: var("a b c x y")
(a, b, c, x, y)
sage: foo = a*x^2 + a*y^2 + b*y^2 + c*y^2 + (2*a*y + b*y)*x
sage: sum([u*foo.coefficient(u) for u in (x^2, y^2, x*y)])
a*x^2 + (2*a + b)*x*y + (a + b + c)*y^2
answer your question ?
HTH,
One way is:
R.<x,y>=SR[]
a,b,c=var('a b c')
p=a*x^2 + a*y^2 + b*y^2 + c*y^2 + (2*a*y + b*y)*x
p
a*x^2 + (2*a + b)*x*y + (a + b + c)*y^2
p.coefficients()
[a, 2*a + b, a + b + c]
Using true polynomials, separating variables, we can ask for coefficients of involved monomials...
R.<a,b,c> = PolynomialRing(QQ)
S.<x,y> = PolynomialRing(R)
f = a*x^2 + a*y^2 + b*y^2 + c*y^2 + (2*a*y + b*y)*x
And now:
for entry in f: print(entry)
gives:
sage: for entry in f: print(entry)
....:
(a, x^2)
(2*a + b, x*y)
(a + b + c, y^2)
So each entry collects the corresponding monomial in $x,y$ in its last component, the first component being the coefficient.
Variant not needing explicit construction of both rings :
sage: var("a, b, c, y")
(a, b, c, y)
sage: f = a*x^2 + a*y^2 + b*y^2 + c*y^2 + (2*a*y + b*y)*x
sage: F=sum([u[0]*u[1] for u in f.polynomial(ring=PolynomialRing(SR,[x, y]))]) ; F
a*x^2 + (2*a + b)*x*y + (a + b + c)*y^2
But beware :
sage: F.parent()
Multivariate Polynomial Ring in x, y over Symbolic Ring
You can get back to SR with this awful trick :
sage: SR(str(F)).parent()
Symbolic Ring
HTH,
Asked: 2023-02-01 09:03:56 +0200
Seen: 124 times
Last updated: Feb 06 '23
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it is too bad that sage can't collect on more than one variable. In Mathematica one can just type
expr = a*x^2 + a*y^2 + b*y^2 + c*y^2 + (2*a*y + b*y)*x; Collect[expr, {x, y}]and geta x^2+(2 a+b) x y+(a+b+c) y^2