## Online Mathematica, pros and cons, linear algebra problem

I apologize in advance if this question is irrelevant to this website.

I would like to use Mathematica to solve a system of linear equations with lots of unknowns(729 unknowns), the unknowns are tensor components of curvature tensors arising from a differential geometry problem.

I would like to buy Mathematica for this purpose and I have to decide between buying it online or installing the desktop version on a PC. I m thinking of buying the online version. I have the following questions:

1. What are the advantages and disadvantages of the desktop version over the online version ? For example, are there mathematical or programming functionalities which are available only on the desktop version and not in the online version ?

2. I assume that if I buy the online version, then I will get a username and a password to access an online version of mathematica from any computer. (Just like how one can type latex on overleaf.com from an online account using any PC). Is my assumption correct ?

3)Does Mathematica provide a user friendly way for solving linear simultaneous equations with lots of unknowns ? Let me elaborate with an example: Say I want to solve the simultaneous equations $$x=2y+a, y-3x=7x+2$$ for $$x,y$$. I would like a software where I can just type: $$x=2y+a, y-3x=7x+2$$ and ask the software to solve for $$x,y$$ and just give me the solution symbolically in terms of parameter $$a$$ instead of me having to rearrange terms so that the equations become $$x-2y=a, y-10x=2$$ and then write it in matrix form, then ask it to make a matrix inversion. The difference I am talking about might seem silly in this example but it will not be silly in my original problem where I have 700 unknowns. If this feature exists in Mathematica, it will save me a lot of time.

Thank you,

## Online SRD vs Official PDF SRD

The question I have is pretty simple. The PDF file that WotC has on their website does not have the same content as this website https://www.5esrd.com/.

If I create a 5e adventure and want to go the OGL route instead of the DMs Guild one, can I use the extra content from the website?

As an example, the website contains a creature named Witchlight. The creature is from the Tome of Beasts, which is a Kobold Press release. What are the restrains there, if any? Reading the title of the website, as it contains SRD in it, it makes me think that I can. But the confusion arises when I don’t see the same content in the WotC file. Can I use this creature and any other that is not in the PDF of WotC?

## Playing online without a VTT, and is it worth it?

New DM here. I have dabbled a bit with D&D in the past, including playing on FG back in 2010. I’m now looking to get back in to it and start up an adventure with my wife and a few friends. My friends are remote, so it would necessitate playing online. I really enjoy rolling actual dice though, and I fear that using a VTT like Roll20 or Fantasy Grounds would kind of cheapen the experience. I’d like to run a pre-written adventure, most likely Mines of Phandelver just to ease the DMing burden rather than create my own adventure right off the bat.

How much more work would it be to just use Discord and basically play without any VTT, doing everything manually? Would this slow down the game too much? I’m eventually shooting for like a 70%/30% RP to combat ratio because I really enjoy theater of the mind vs. tactical maps/dungeon crawls. Is this insane? Should I just use a VTT and get on with it?

## Want to sell images online on wordpress but slow image loading issues

I wish to sell images, videos and music online but if I add the media files in WordPress, the website becomes slow. IS it possible to add a library? where I can upload thousands of images and videos without slowing down my WordPress site and linking it to the website?

## pgpool-II and Postgres docker image : automated failover and online recovery via rsa key

I’ve been following this documentation for pgpool-ii https://www.pgpool.net/docs/latest/en/html/example-cluster.html

I’m having a hard time setting up rsa on my postgres streaming cluster built in official docker image https://hub.docker.com/_/postgres.

I was able to do the streaming now i’m on the part of setting up failover.

part of the documentation says.

``To use the automated failover and online recovery of Pgpool-II, the settings that allow passwordless SSH to all backend servers between Pgpool-II execution user (default root user) and postgres user and between postgres user and postgres user are necessary. Execute the following command on all servers to set up passwordless SSH. The generated key file name is id_rsa_pgpool.  ``
``     [all servers]# cd ~/.ssh      [all servers]# ssh-keygen -t rsa -f id_rsa_pgpool      [all servers]# ssh-copy-id -i id_rsa_pgpool.pub postgres@server1      [all servers]# ssh-copy-id -i id_rsa_pgpool.pub postgres@server2      [all servers]# ssh-copy-id -i id_rsa_pgpool.pub postgres@server3       [all servers]# su - postgres      [all servers]\$   cd ~/.ssh      [all servers]\$   ssh-keygen -t rsa -f id_rsa_pgpool      [all servers]\$   ssh-copy-id -i id_rsa_pgpool.pub postgres@server1      [all servers]\$   ssh-copy-id -i id_rsa_pgpool.pub postgres@server2      [all servers]\$   ssh-copy-id -i id_rsa_pgpool.pub postgres@server3 ``

Is it possible to set it up inside a container from postgre’s official image? I would like to get an idea on how to do it from some samples or existing solution.

Moreover, Since I can’t do the `rsa` thing as of the moment.

I decided to create a script that is using a `psql` command on my pgpool server to the new master

``#!/bin/bash # This script is run by failover_command.  set -e  # Special values: #   %d = failed node id #   %h = failed node hostname #   %p = failed node port number #   %D = failed node database cluster path #   %m = new master node id #   %H = new master node hostname #   %M = old master node id #   %P = old primary node id #   %r = new master port number #   %R = new master database cluster path #   %N = old primary node hostname #   %S = old primary node port number #   %% = '%' character  FAILED_NODE_ID="\$  1" FAILED_NODE_HOST="\$  2" FAILED_NODE_PORT="\$  3" FAILED_NODE_PGDATA="\$  4" NEW_MASTER_NODE_ID="\$  5" NEW_MASTER_NODE_HOST="\$  6" OLD_MASTER_NODE_ID="\$  7" OLD_PRIMARY_NODE_ID="\$  8" NEW_MASTER_NODE_PORT="\$  9" NEW_MASTER_NODE_PGDATA="\$  {10}" OLD_PRIMARY_NODE_HOST="\$  {11}" OLD_PRIMARY_NODE_PORT="\$  {12}"  #set -o xtrace #exec > >(logger -i -p local1.info) 2>&1  new_master_host=\$  NEW_MASTER_NODE_HOST ## If there's no master node anymore, skip failover. if [ \$  NEW_MASTER_NODE_ID -lt 0 ]; then     echo "All nodes are down. Skipping failover."     exit 0 fi  ## Promote Standby node. echo "Primary node is down, promote standby node" \$  {NEW_MASTER_NODE_HOST}.  PGPASSWORD=postgres psql -h \$  {NEW_MASTER_NODE_HOST} -p 5432 -U postgres <<-EOSQL  select pg_promote(); EOSQL  #logger -i -p local1.info failover.sh: end: new_master_node_id=\$  NEW_MASTER_NODE_ID started as the primary node #exit 0 ``

The above script is working if i simulate that my primary is down.

However, in my new primary this is the log

``2020-10-07 20:25:31.924 UTC [1165] LOG:  archive command failed with exit code 1 2020-10-07 20:25:31.924 UTC [1165] DETAIL:  The failed archive command was: cp pg_wal/00000002.history /archives/00000002.history cp: cannot create regular file '/archives/00000002.history': No such file or directory 2020-10-07 20:25:32.939 UTC [1165] LOG:  archive command failed with exit code 1 2020-10-07 20:25:32.939 UTC [1165] DETAIL:  The failed archive command was: cp pg_wal/00000002.history /archives/00000002.history 2020-10-07 20:25:32.939 UTC [1165] WARNING:  archiving write-ahead log file "00000002.history" failed too many times, will try again later cp: cannot create regular file '/archives/00000002.history': No such file or directory 2020-10-07 20:26:33.003 UTC [1165] LOG:  archive command failed with exit code 1 2020-10-07 20:26:33.003 UTC [1165] DETAIL:  The failed archive command was: cp pg_wal/00000002.history /archives/00000002.history cp: cannot create regular file '/archives/00000002.history': No such file or directory 2020-10-07 20:26:34.012 UTC [1165] LOG:  archive command failed with exit code 1 2020-10-07 20:26:34.012 UTC [1165] DETAIL:  The failed archive command was: cp pg_wal/00000002.history /archives/00000002.history cp: cannot create regular file '/archives/00000002.history': No such file or directory 2020-10-07 20:26:35.026 UTC [1165] LOG:  archive command failed with exit code 1 2020-10-07 20:26:35.026 UTC [1165] DETAIL:  The failed archive command was: cp pg_wal/00000002.history /archives/00000002.history 2020-10-07 20:26:35.026 UTC [1165] WARNING:  archiving write-ahead log file "00000002.history" failed too many times, will try again later cp: cannot create regular file '/archives/00000002.history': No such file or directory 2020-10-07 20:27:35.096 UTC [1165] LOG:  archive command failed with exit code 1 2020-10-07 20:27:35.096 UTC [1165] DETAIL:  The failed archive command was: cp pg_wal/00000002.history /archives/00000002.history cp: cannot create regular file '/archives/00000002.history': No such file or directory 2020-10-07 20:27:36.110 UTC [1165] LOG:  archive command failed with exit code 1 2020-10-07 20:27:36.110 UTC [1165] DETAIL:  The failed archive command was: cp pg_wal/00000002.history /archives/00000002.history cp: cannot create regular file '/archives/00000002.history': No such file or directory 2020-10-07 20:27:37.123 UTC [1165] LOG:  archive command failed with exit code 1 2020-10-07 20:27:37.123 UTC [1165] DETAIL:  The failed archive command was: cp pg_wal/00000002.history /archives/00000002.history 2020-10-07 20:27:37.123 UTC [1165] WARNING:  archiving write-ahead log file "00000002.history" failed too many times, will try again later cp: cannot create regular file '/archives/00000002.history': No such file or directory 2020-10-07 20:28:37.177 UTC [1165] LOG:  archive command failed with exit code 1 2020-10-07 20:28:37.177 UTC [1165] DETAIL:  The failed archive command was: cp pg_wal/00000002.history /archives/00000002.history cp: cannot create regular file '/archives/00000002.history': No such file or directory 2020-10-07 20:28:38.221 UTC [1165] LOG:  archive command failed with exit code 1 2020-10-07 20:28:38.221 UTC [1165] DETAIL:  The failed archive command was: cp pg_wal/00000002.history /archives/00000002.history cp: cannot create regular file '/archives/00000002.history': No such file or directory 2020-10-07 20:28:39.230 UTC [1165] LOG:  archive command failed with exit code 1 2020-10-07 20:28:39.230 UTC [1165] DETAIL:  The failed archive command was: cp pg_wal/00000002.history /archives/00000002.history 2020-10-07 20:28:39.230 UTC [1165] WARNING:  archiving write-ahead log file "00000002.history" failed too many times, will try again later ``

still trying to execute the WAL part.

moreover, on my other standby it is still looking for the old master.

``2020-10-07 20:29:07.818 UTC [1365] FATAL:  could not connect to the primary server: could not translate host name "pg-1" to address: Name or service not known 2020-10-07 20:29:12.827 UTC [1367] FATAL:  could not connect to the primary server: could not translate host name "pg-1" to address: Name or service not known 2020-10-07 20:29:17.832 UTC [1369] FATAL:  could not connect to the primary server: could not translate host name "pg-1" to address: Name or service not known 2020-10-07 20:29:22.835 UTC [1371] FATAL:  could not connect to the primary server: could not translate host name "pg-1" to address: Name or service not known 2020-10-07 20:29:27.826 UTC [1373] FATAL:  could not connect to the primary server: could not translate host name "pg-1" to address: Name or service not known 2020-10-07 20:29:32.836 UTC [1375] FATAL:  could not connect to the primary server: could not translate host name "pg-1" to address: Name or service not known 2020-10-07 20:29:37.836 UTC [1377] FATAL:  could not connect to the primary server: could not translate host name "pg-1" to address: Name or service not known 2020-10-07 20:29:42.850 UTC [1379] FATAL:  could not connect to the primary server: could not translate host name "pg-1" to address: Name or service not known 2020-10-07 20:29:47.857 UTC [1381] FATAL:  could not connect to the primary server: could not translate host name "pg-1" to address: Name or service not known 2020-10-07 20:29:52.855 UTC [1383] FATAL:  could not connect to the primary server: could not translate host name "pg-1" to address: Name or service not known ``

and dealing with this I think is more complicated than setting up the rsa part so that i could utilize the existing fail_command script that pgpool has.

Thanks for the response.

## Online Lottery Affiliate Website

Why are you selling this site? Too many projects on the go.

How is it monetized? Lotto affiliate links

Does this site come with any social media accounts? NO

How much time does this site take to run? Nothing to add, but promoting

What challenges are there with running this site? Lotto sites are banned in some countries, so it requires targeting

I…

Online Lottery Affiliate Website

## Online VAT Calculator – Vatulator

Why are you selling this site?
Need money

How is it monetized?
Not yet monetized

Does this site come with any social media accounts?
No

How much time does this site take to run?
It depends

What challenges are there with running this site?
Nothing

## Configuration to use OWASP WebGoat whilw online? [closed]

OWASP WebGoat is a deliberately insecure webapplication with a set of tutorials how to hack it (and how to protect your own application). OWASP advises to disconnect from the internet while using it since it is an insecure application after all, and firewalls might be misconfigured or missing (see the prior question OWASP WebGoat Warning Meaning).

Is there a checklist how to use WebGoat safely while in a video conference? The first step is presumably to run it in the docker container. How do I configure that docker?

## CLRS 22.3-1, How Come Solutions Online State There Can’t Be Edges From WHITE to GRAY nodes “at any point… during search”?

The exercise (from the book Introduction To Algorithms) states

Make a 3-by-3 chart with row and column labels WHITE, GRAY,and BLACK. In each cell (i, j) indicate whether, at any point during a depth-first search of a di- rected graph, there can be an edge from a vertex of color i to a vertex of color j . For each possible edge, indicate what edge types it can be. Make a second such chart for depth-first search of an undirected graph.

The colors WHITE, GRAY, BLACK correspond to Undiscovered, discovered but not finished, and finished. The following solution is what multiple sites & universities have posted(such as: walkccc, Rutgers University):

`` |       | WHITE         | GRAY                | BLACK                | |-------|---------------|---------------------|----------------------| | WHITE | All kinds     | Cross, Back         | Cross                | | GRAY  | Tree, Forward | Tree, Forward, Back | Tree, Forward, Cross | | BLACK | -             | Back                | All kinds            | ``

I will draw a minimal counter example as it helps understand my conflict:

• Start at node 0: 0 is GRAY
• PAUSE
• At this point, 3 is still white and has an edge to 0
• Resume and keep going, eventually the edge from 3 to 0 will be discovered as a tree edge

This contradicts the solutions saying you can only have Cross/Back edges going form WHITE->GRAY. This example can be easily modified to contradict many of the elements in the table. I think the solutions are doing one of the following:

• Assuming that the graph is a tree and that we start at its root. (Wrong as DFS doesn’t need a tree graph and any node can be started from).
• More likely (Just thought of this), interpreting the question of "can there be an edge" as "can there be an edge that we have discovered". In which case, the solutions work, as although the edge from 3->0 was a WHITE->GRAY edge at one point, we hadn’t discovered it yet.