[GSoC] Added symbolic classes for Moment and Centralmoment

[Smit-create]

References to other Issues or PRs

Fixes #6323
Related comment #6323 (comment)

Brief description of what is fixed or changed

• Add classes of Moment and CentralMoment and their docs
• Tests

Other comments

Release Notes

• stats
  • Added symbolic classes of Moment and CentralMoment

ping @Upabjojr @czgdp1807 @jmig5776

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• stats
  • Added symbolic classes of Moment and CentralMoment (#19724 by @Smit-create)

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#### References to other Issues or PRs
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Fixes #6323 
Related comment https://github.com/sympy/sympy/issues/6323#issuecomment-654743869

#### Brief description of what is fixed or changed
- [x] Add classes of Moment and CentralMoment and their docs
- [x] Tests

#### Other comments


#### Release Notes

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* stats
   *  Added symbolic classes of `Moment` and `CentralMoment`
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ping @Upabjojr @czgdp1807 @jmig5776 

Update

The release notes on the wiki have been updated.

[Smit-create]

@Upabjojr @czgdp1807 I get the following results on the current diff, Please see if these are the expected results:

In [1]: from sympy.stats import *

In [2]: X = Normal('X', 1, 2)

In [3]: M = Moment(X, 2, 3)

In [4]: M
Out[4]: Moment(X, 2, 3)

In [5]: M.doit()
Out[5]: 8

In [6]: M.rewrite(Expectation)

Out[6]: Expectation((X - 3)**2)

In [7]: M.rewrite(Probability)
Out[7]:
∞
⌠
⎮         2
⎮  (x - 3) ⋅Probability(Eq(X, x)) dx
⌡
-∞

In [8]: M.rewrite(Integral)
Out[8]:
∞
⌠
⎮                       2
⎮               -(X - 1)

⎮               ──────────
⎮            2      8
⎮  √2⋅(X - 3) ⋅ℯ
⎮  ─────────────────────── dX
⎮            4⋅√π
⌡
-∞

In [9]: m = moment(X, 2, 3)

In [11]: m
Out[11]: 8

In [12]: m = moment(X, 2, 3, evaluate=False)

In [13]: m
Out[13]:
∞
⌠
⎮                       2
⎮               -(X - 1)
⎮               ──────────
⎮            2      8
⎮  √2⋅(X - 3) ⋅ℯ
⎮  ─────────────────────── dX
⎮            4⋅√π
⌡
-∞


In [14]: M.rewrite(Expectation).expand()
Out[14]: 9 - 6⋅Expectation(X) + Expectation(X**2)

In [15]: M.rewrite(Expectation).expand().doit()
Out[15]: 8

[codecov]

Codecov Report

Merging #19724 into master will decrease coverage by 11.315%.
The diff coverage is 91.111%.

@@              Coverage Diff               @@
##            master    #19724        +/-   ##
==============================================
- Coverage   75.707%   64.391%   -11.316%     
==============================================
  Files          659       659                
  Lines       171329    171376        +47     
  Branches     40415     40422         +7     
==============================================
- Hits        129709    110352     -19357     
- Misses       35977     54741     +18764     
- Partials      5643      6283       +640     

[Smit-create]

@Upabjojr @czgdp1807 Please have a look if this looks good to merge.

[czgdp1807]

Looks okay.

[Upabjojr]

It looks ok to me as well.