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Questions about the function Assuming[], the $Assumptions system variable, and the Assumptions option of functions such as Simplify[] and Integrate[].
21
votes
Accepted
Logarithm of exponential
Another method is to use PowerExpand:
PowerExpand[Log[E^a], Assumptions -> True]
a + 2 I π Floor[1/2 - Im[a]/(2 π)]
Addendum
As an aside, Log behave exactly like ArcSin here:
ArcSin[Sin[x]] … Pi/2 < x < Pi/2]
x
For larger domains, Simplify doesn't work:
Simplify[ArcSin[Sin[x]], 0 < x < 2 Pi]
ArcSin[Sin[x]]
Again, using PowerExpand is useful:
p = PowerExpand[ArcSin[Sin[x]], Assumptions …
7
votes
Accepted
Representing number domain by a symbol
Per the documentation you can use Esc ints Esc to enter the StandardForm symbol for Integers
7
votes
Defining the domain of positive real numbers
New in Mathematica 12 is PositiveReals (and others like NonNegativeIntegers, etc):
Solve[x^2 == 1, x, PositiveReals]
{{x -> 1}}
5
votes
Accepted
Laurent Series with Symbolic Powers
One possibility is to just replace your variable with one whose powers are all explicit integers, and then use Series:
e = PowerExpand[
a^(-c2)/((a*c1)^(c2) + c3) /. a -> z^(1/c2),
Assumptions …
5
votes
Accepted
Mathematica does not respect tensor order?
Note what happens when you add them:
$Assumptions = Element[d, Matrices[{3, 3}, Reals, Symmetric[{1,2}]]];
d + IdentityMatrix[3]
{{1 + d, d, d}, {d, 1 + d, d}, {d, d, 1 + d}}
The Listable attribute …
5
votes
Accepted
How to 'define' value in Assumptions but not let Simplify replace it?
One possibility is to mimic the behavior of symbolic constants like Pi, E, etc:
N[x, _] ^= 4;
NumericQ[x] ^= True;
Then:
Sqrt[x^2]
x
without even using Simplify.
4
votes
how to make DSolve not generate Assumptions in the solution?
TrueQ@$IntFlag := Block[{$Foo=True},
MakeBoxes[Integrate[a],TraditionalForm]
]
Protect[Integrate];
Then:
Integrate[f[x]Sin[x],{x,0,L0},Assumptions->True] //TeXForm
$\int_0^{\operatorname{L0 …
4
votes
Accepted
Solve yields complex Root even if the domain is Reals
ToRadicals supports the undocumented option Assumptions. … Making use of this option gives:
sol = Solve[x^3==a^2,x,Reals]
ToRadicals[sol, Assumptions -> a>0]
{{x -> Root[-a^2 + #1^3 &, 1]}}
{{x -> a^(2/3)}} …
4
votes
Accepted
Taking residues with integer assumptions?
You could use SeriesCoefficient instead:
Assuming[n ∈ Integers && n>1,
Simplify @ SeriesCoefficient[
Exp[x] x^(-1-n),
{x, 0, -1}
]
]
1/n!
4
votes
How to define an antisymmetric symbol?
For example:
TensorReduce[
w + TensorTranspose[w, {1, 3, 2}],
Assumptions -> w ∈ Arrays[{n, n, n}, Complexes, Antisymmetric[{1,2,3}]]
]
0 …
3
votes
Simplifying inequalities with exponents
I think using Reduce would be a better approach, although it's not completely straightforward:
Reduce[x^a < 1 && a > 0 && x > 1, Reals]
False
(updated with another approach using Resolve)
Ano …
3
votes
Accepted
How to Integrate the Derivative of a function with Assumptions on the function?
You could try using DSolveValue. First, define your sum using inactive integrals:
h[α_] := Inactive[Integrate][x g'[x],{x,0,α}] + Inactive[Integrate][α g'[x],{x,α,∞}]
Even though the integrals are …
3
votes
Accepted
Triple integral: How do I include assumptions and conditions?
The region of integration is:
reg = ImplicitRegion[0 < y < a && 0 < x < z && 0 < z < y, {x, y, z}];
Using this region in Integrate:
sol = Integrate[Exp[(a-x)^3], {x, y, z} ∈ reg, Assumptions -> a > 0] …
3
votes
Accepted
Extracting a logarithmic divergence of an expression using Series
Another possibility is to use the new in M12.1 function Asymptotic:
asym = Assuming[
r0 > 3 M > 0 && θ > Pi/2,
Simplify @ Asymptotic[expr, θ -> Pi/2]
]
Coefficient[asym, Log[θ - Pi/2]] //Simpl …
3
votes
Usage of Assuming for Integration
For this example, you could use FourierCoefficient instead of Integrate:
FourierCoefficient[1, t, k, FourierParameters->{-1,1}]
2 π DiscreteDelta[k]