Trouble implementing outflow boundary condition when trying to solve a pde using NDSolve

Trying to solve the following pde: $ \partial_{t}y + c\partial_{c}y = 0$ (for simplicity $ c=1$ ).

For the initial data I am using a Gaussian. The problem surges when I am trying to implement the outflow boundary condition as it was suggested to me, namely $ \frac{\partial{y}}{\partial{t}} =0$ at $ x =0$ .

So far my code is pretty simple:

 v = 1 ; L = 2;   With[{y = y[t, x]},   eq = D[y, t] + vD[y, x] == 0;  ic = y == Exp[-x^2] /. t -> 0;  bc = {D[y, x] == 0 /. x -> 0 }];    mol[n_Integer, o_: "Pseudospectral"] := {"MethodOfLines",   "SpatialDiscretization" -> {"TensorProductGrid", "MaxPoints" -> n,   "MinPoints" -> n, "DifferenceOrder" -> o}};   sol = NDSolveValue[{eq, ic, bc}, y, {t, 0, 1}, {x, 0, L},   Method -> mol[100, 4]];  {t0, tend} = sol["Domain"][[1]];   Manipulate[  Plot[sol[t, x], {x, 0, L}, PlotRange -> {-10, 10}], {t, 0, tend}]; 

It stems from answers to questions previously asked here, and I intend later to test finite difference methods (BTCS/FTCS) as done in Schemes for nonlinear advection equation.

However I am not being able to evolve the equation do to confusion when trying to implement the BC, I get the following:

  NDSolveValue: Boundary condition $  y^{(0,1)}[t,0]$   should have derivatives of order lower than the differential order of the partial    differential equation.  

This is expected as I am not sure what would be the best way to impose BC on the problem.

If anyone has any suggestions they would be welcolmed.


Does a barbarian’s rage end if they are trying to attack an invisible creature they don’t know the location of?

The barbarian rage ability includes this rule stating when it will end:

Your rage lasts for 1 minute. It ends early if you are knocked unconscious or if your turn ends and you haven’t attacked a hostile creature since your last turn or taken damage since then.

What happens if the barbarian sees an enemy turn invisible during a battle and begins wildly swinging their weapon at seemingly empty air? From the point of view of the barbarian the enemy might be in the location they are trying to attack. Would that be enough to continue their rage, or would they need to actually hit their enemy or be certain of their location before attacking?

I am trying to make a homebrew campaign. I have been trying to brainstorm the idea for the start. Is this good enough?

The player’s characters were hired by Tulmund Bilsh, a silk merchant from Alablast, a diverse town which also happens to be the player character’s home town, to help escort him and his goods to a primarily human city, Vern, and will pay the players once the journey has been completed.

They begin in a tavern in a small swamp village named Stillwater, where they plan to rest and stay the night. Right before they hit the hay, though, the village is attacked by a tribe of lizardfolk, and Tulmund disappears during the attack. With enough questioning of the locals, the players will find out that lizardfolk in this region are nearing extinction, and they lurk in the ruins of a once proud city, the only building intact being a temple.

Assuming the players still want their pay, they will assault the temple, and fight their way through brainwashed villagers, strange constructs and a few lizardfolk sprinkled in. They find a multitude of cells along their way, but all of them empty or containing corpses, none of which are that of the silk merchant’s. They finally fight their way into a large room with an altar at the center, with a strange blue orb sitting on a pedestal. They find the silk merchant being forced to his knees, facing towards the orb by two brainwashed villagers as a blue mist begins to emit from the orb and make it’s way towards the merchant, this is presumably the way the lizardfolk brainwash people. A robed lizardfolk stands behind the altar, chanting and the players will engage combat with two constructs, some lizardfolk and some other brainwashed people.

If the players take out the ones holding Tulmund, the orb is taken off the pedestal or interrupt the robed lizardfolk, Tulmund will not be brainwashed. If the orb is taken off the pedestal, the altar will open and a shadowy aberration will begin to rise from the altar and the players will have to get 13+ on a wisdom saving throw or be inflicted with Crown of Madness.

Yes, I am aware that this is a sloppy mess and does require more finer details, but this is the general idea I had to start out my campaign without it being a basic “you start in a tavern” kind of deal.

why is update_user_meta creating arrays when trying to change the same value?

I added a custom field using update_user_meta to count the products user can buy. Initially the counter starts at 10 and after buying each product the field is deducted by the number of products user bought. Here is the code

function deduct_product($  order_id){   $  order=wc_get_order($  order_id);   $  items=$  order->get_items();   $  quantity=0;   foreach($  items as $  item_id => $  item_data){     $  quantity+=$  item_data->get_quantity();   }   $  remaining_products=get_user_meta(get_current_user_id(),'remaining_products');     $  remaining_products-=$  quantity;   update_user_meta(get_current_user_id(  ),'remaining_products',$  remaining_products); } 

It gives me error that unsupported operand types on line where i have used $ remaining_products-=$ quantity. How do I change the user meta for remaining_products by the number of products user has bought.

Trying to contact Red Orca for permission to print an answer in a book [migrated]

I apologize in advance for breaking protocol. I’m just looking to get an official okay on rewriting and publishing Red Orca’s answer in this thread in an upcoming book about Magic Items. I looked for alternative ways to contact them but fell short. It’s an incredible answer and I’d love to make it more widely known to people out there looking to categorize their own items.

They can reach out over reddit to u/griff-mac or Discord to griffmac#9421, if that’s more appropriate. Thanks in advance, and again, I hope I’m not breaking rules too egregiously.


What’s the RPG about a journey from A to B, with examples about French soldiers going to kill Hitler and a team trying to win a TV race contest?

There’s this roleplaying game that’s all about a group of people trying to get from point A to point B.

The two example scenarios that are the base for all the examples in the manual are French soldiers going to Berlin to kill Hitler and guys on a TV game show needing to reach point B to win a prize before a team of hunters catches them.

If I’m not mixing two different games together, the main game mechanic is throwing dice from a certain distance at a target lying flat on a table. Rolling outside the target (too long or too short) or knocking over the pile of dice stacked in the center of the target means failure.

Is there a rules-consistent way to rule a character trying to grab a snake?

Bad-guy sorceror just cast Summon Monster I and now a Snake has appeared next to a player. The snake is right next to the king, so she wants to make a go at grabbing the snake and cutting off it’s head.

Is there a rules-based way for me work out the roll and DC? I was thinking it’s not a Grapple check, as she’s not really wrestling against a small snake, so maybe it’s an opposed DEX check?

How likely it is that a hacker spoofed my own IP address when trying to log in to a website?

I got a confirmation code without trying to log in which is what prompted me to investigate this. The account is just a Ubisoft account with 1 free game on it; I somehow doubt there would be motivation to employ sophisticated hacker techniques.

I know about Tor services and VPN, but I don’t think those would spoof my own IP address. When I look at the log-in history for my account, it says my very own IP (at least, an IP with the same first and last segment) failed a login attempt 15 minutes ago and I definitely don’t remember doing anything remotely related to that account in the last 12 hours. Did a hacker actually spoof the correct first and last numbers of my IP, or is this more likely some sort of bug? How would they even know the right IP to spoof in the first place

Can I surf the internet with a Virtual Machine not trying to malware test but may run into some

For this project I want to access software using a VM that may contain malware but I won’t be trying to malware test. I understand that malware can breach but are there any precautions to use the internet and not have the malware come back into my host machine and host network?

Could you anonymously upload a file on the internet if the threat model was the entire world trying to find your identity after you do so?

Thought experiment: You need to upload a file, and the threat model is the entire world trying to find out who you are after you do so.

I know this is absurd, but bear with me, it’s a thought experiment that I thought would be fun to have, where the scenario is the following:

You are a normal citizen, and you have a file (assume that you just have it, and the file doesn’t have metadata or information related to you) that is somehow so compromising that, if uploaded on the internet, the entire world would actively try to find out who you are. Everyone, military, every country’s agencies, civilians, that grandma going to the grocery store, yes, her too, to the best of her ability. People who run TOR relays too, everyone.

Your mission is to upload it on the internet without your identity being revealed. How would you go about it?

Update: File is in your pendrive, has a size of few MB, it’s ok if it just shows up eventually. No-one should really guess what country you might live in. We can think of the user as average-citizen, with average knowledge of technology, but can follow instructions (for example, setting up Tails). Assume that who had access to this information was not relevant, because no-one had access to this data before. We can invent an unrealistic scenario for this, for example, you have material that prove the existence of aliens or something.