Add a "signless" option to laplacian

[jasongrout]

We should have an option to g.laplacian() to return the signless Laplacian, which is attracting attention these days, and which is calculated by D+A instead of D-A (see p. 12 of http://www.doiserbia.nb.rs/ft.aspx?id=0350-13020795011C, for example).

Thanks to Steve Butler for the feature request.

CC: @nathanncohen @rlmill @dcoudert

Component: graph theory

Author: Rajat Mittal

Branch/Commit: e2b64c6

Reviewer: David Coudert

Issue created by migration from https://trac.sagemath.org/ticket/9707

[rajat1433]

comment:5

Signless laplacian is indeed gaining popularity as evident in the papers below.
So can I add the option to the current Laplacian Matrix method to return signless laplacian matrix?

https://arxiv.org/pdf/1803.06135.pdf

http://elib.mi.sanu.ac.rs/files/journals/publ/101/n095p011.pdf

https://ac.els-cdn.com/S0024379507000316/1-s2.0-S0024379507000316-main.pdf?_tid=59a3915e-dd7a-4dea-87a7-1892bc82cdca&acdnat=1552416165_1a56db5226e8357d8b7c9879a5dc3973

[dcoudert]

comment:6

A quick search effectively returns a significant number of recent publications.

[rajat1433]

Reviewer: dcoudert

[rajat1433]

Author: Rajat Mittal

[rajat1433]

Branch: u/gh-rajat1433/9707_signless_laplacian

[sagetrac-git]

Commit: e2b64c6

[sagetrac-git]

Branch pushed to git repo; I updated commit sha1. New commits:

e2b64c6

added signless option

[rajat1433]

comment:10

Do i need to include this ticket number somewhere in the code?

[dcoudert]

comment:11

We usually add ticket number when we fix a bug. So it's not needed here.

LGTM.

[dcoudert]

Changed reviewer from dcoudert to David Coudert

[vbraun]

Changed branch from u/gh-rajat1433/9707_signless_laplacian to e2b64c6