10.30.1 Top new questions this week

10.30.1.1 NSolve uses all CPU resources
10.30.1.2 Solving a matrix differential equation with change of variable
10.30.1.3 How to make conditions work in Which?
10.30.1.4 Why does DiagonalizableMatrixQ give different answers when Simplify or FullSimplify is applied to a 4x4 matrix, consisting of complex trig. functions?
10.30.1.5 Solve cannot find solutions if integer parameters are assumed
10.30.1.6 Transform a list of rules into a list of function definitions
10.30.1.7 Sum[] function not computing the sum
10.30.1.1 NSolve uses all CPU resources

https://mathematica.stackexchange.com/landing/r/digest?cta=question&id=306871

Is it default setting of  NSolve  that it uses all my four cores of CPU? 
Be aware that the following code made my computer non-respondent for about 18 seconds till it ... 
[equation-solving] [performance-tuning] [numerics] [system-performance]
 
asked by azerbajdzan https://mathematica.stackexchange.com/landing/r/digest?cta=user&id=53172 Score of 7
answered by Daniel Lichtblau https://mathematica.stackexchange.com/landing/r/digest?cta=user&id=51 Score of 8
10.30.1.2 Solving a matrix differential equation with change of variable

https://mathematica.stackexchange.com/landing/r/digest?cta=question&id=306886

In order to solve $\partial_t X(t) = -i(M X(t) - X(t) M^*)$ (where * is conjugate-transpose), I wrote the following code: 
... 
[differential-equations] [equation-solving] [matrix]
 
asked by phy_std https://mathematica.stackexchange.com/landing/r/digest?cta=user&id=98622 Score of 4
10.30.1.3 How to make conditions work in Which?

https://mathematica.stackexchange.com/landing/r/digest?cta=question&id=306859

How to make Condition convert to common expresstions when it mathes Which test. 
... 
[condition]
 
asked by eason https://mathematica.stackexchange.com/landing/r/digest?cta=user&id=73017 Score of 4
answered by Domen https://mathematica.stackexchange.com/landing/r/digest?cta=user&id=75628 Score of 5
10.30.1.4 Why does DiagonalizableMatrixQ give different answers when Simplify or FullSimplify is applied to a 4x4 matrix, consisting of complex trig. functions?

https://mathematica.stackexchange.com/landing/r/digest?cta=question&id=306761

NOTE: I HAVE CLEANED UP THIS DISCUSSION TO MAKE THINGS MORE TRANSPARENT: 
I am using Mathematica version 12.1. I have a 4x4 matrix called  matrixA[x, y] , which ... 
[matrix] [bugs]
 
asked by Quantum Kid https://mathematica.stackexchange.com/landing/r/digest?cta=user&id=95345 Score of 4
10.30.1.5 Solve cannot find solutions if integer parameters are assumed

https://mathematica.stackexchange.com/landing/r/digest?cta=question&id=306742

This is a very simple toy problem that illustrates the problem. 
Start with a fresh kernel. 
 Quit[] 
 
Make some simple assumptions 
... 
[equation-solving] [special-functions]
 
asked by Bill Watts https://mathematica.stackexchange.com/landing/r/digest?cta=user&id=53121 Score of 4
answered by Michael E2 https://mathematica.stackexchange.com/landing/r/digest?cta=user&id=4999 Score of 3
10.30.1.6 Transform a list of rules into a list of function definitions

https://mathematica.stackexchange.com/landing/r/digest?cta=question&id=306745

I have a list of rules obtained by solving a system of equations, e.g. 
 {f[x] -> a + 3 h[x], g[x] -> 2 a + c + 6 h[x]} . 
I wish to convert this list of rules ... 
[functions] [replacement] [assignment]
 
asked by Stanislas Antczak https://mathematica.stackexchange.com/landing/r/digest?cta=user&id=101570 Score of 4
answered by Craig Carter https://mathematica.stackexchange.com/landing/r/digest?cta=user&id=12461 Score of 5
10.30.1.7 Sum[] function not computing the sum

https://mathematica.stackexchange.com/landing/r/digest?cta=question&id=306763

I have the following sum: 
$$\sum _{y=0}^x \left(1-n^2! \left(n^2\right)^{-y} \mathcal{S}_y^{\left(n^2\right)}\right)$$ 
Which, in Wolfram language can be written as: 
... 
[calculus-and-analysis]
 
asked by Cardstdani https://mathematica.stackexchange.com/landing/r/digest?cta=user&id=98456 Score of 4
answered by azerbajdzan https://mathematica.stackexchange.com/landing/r/digest?cta=user&id=53172 Score of 8