## dharr

Dr. David Harrington

## 6337 Reputation

19 years, 352 days
University of Victoria
Professor or university staff

## Social Networks and Content at Maplesoft.com

I am a professor of chemistry at the University of Victoria, BC, Canada, where my research areas are electrochemistry and surface science. I have been a user of Maple since about 1990.

## solve, identity...

@salim-barzani Solve(identity(... takes an equation, not multiple equations as CoefficientNullity is.

So you could use

`solve(identity(J, xi), {k, w, A[0], A[1], B[1], a[1], a[2], a[3], a[4], a[5]})`

or (probably faster)

`solve(identity(DEq1_1, Z), {k, w, A[0], A[1], B[1], a[1], a[2], a[3], a[4], a[5]})`

@salim-barzani So do it in small parts. A2 looks right here but how can A1 have f(something) + f(something) if it is f(sum)? Maybe that is what the seq was for (should have been add), but if that is the case then A2 can't be right.

Very confusing.

Mp.mw

## seq...

Your seq(expr, m=1..2) just makes two copies of expr, so you have f(expr, expr). Maple ignores the second argument so you just get f(expr). Aside from that it does what you ask it. eval(something, lambda=0) is preferred over subs(lambda=0, something) as it it mathematically smarter, though in this case it doesn't make any difference.

## combining properties...

@salim-barzani you can use And and Or to combine properties, e.g.
assuming Or(positive, negative); [all variables real and nonzero]
assuming Or(x::positive, x::negative) [x real and nonzero]

assuming x<>0 [x non-zero but could be complex]

See the help pages ?assuming  and ?assuming,details

## no real reason...

@Nicole Sharp I kept 2015 as it is an institutional licence that my students had access to. But 2022 and 2023 were just licenced to me, and I kept 2022 when I updated to 2023 "just in case". Now I have updated to 2024 on a different computer and will soon junk the old computer and all three earlier versions. Decreases in capability are very rare. I don't know anything about Quandl, but it seems the DataSets package has just changed so similar data is now built-in to Maple, so there is perhaps not much decrease in capability?

## multiple versions...

In general, there is no problem having multiple versions - I have a windows 10 system with 2015, 2022 and 2023 happily coexisting.

## arclength...

@one man With apologies to Rouben, I hacked his code to get the arclength, which for his solution agrees approximately with your second solution. Is your first one just symmetrically equivalent with not very accurate arclength? It might be possible to get such solutions from Rouben's code by givng dsolve an approximate initial solution.

geodesic-on-rounded-cube-arclength.mw

On Maple 2023, if I have a worksheet open I see Sign in at the top right corner. I sign in and on the sign-in window there is a "Remember me" check box. If I check this, then I am persistently signed in; my avatar shows when I open a new worksheet. So perhaps it is some sort of access/permissions issue on your computer. (I don't have Maple 2024 at my location right now.)

## workaround...

@zenterix I think @C_R has the solution for the g=1 case, which was just your test case. For the physically realistic case of g>1, it can be done via the symbolic solution. But for numerical integration I think it has to be non-dimensionalized along @C_R's lines ,since V^g is problematic with g=1.1 - see end of worksheet.

Edit: updated to correct incorrect assumption about temperature

Edit: like many derivations the derivation of P*V^g = k involved raising something with units to a non integral power, which is the root cause of the problem here. So probably the derivation can be improved by adding some variables that are just there to cancel the units, e.g., (V/V0)^g with V0 = 1 m^3, similar to @C_R's treatment . But then of course V0 will pop up somewhere else in k.

## comment...

@Scot Gould Yes, I find this sort of thing frustrating and have to rediscover it frequently when I want an unsual data structure. I think as a rule, Arrays are more permissive about what you put into them, but for Matrices and Vectors, elements that are Vectors/Matrices/lists get incorporated into a single Matrix/Vector of numbers/symbols.

## meaning of n?...

In Maple 2024, I get a warning "Warning, units problem, not enough information to unambiguously deduce the units of the variables {V, n}; proceeding as if dimensionless"

in the integration limits you have n times a unit being a volume, which seems strange. Do you mean something like V = n*V1..m*V1 where n is dimensionless and V1 is a volume. Or did you mean n and m to have values before the integral?

## plane?...

`plot3d(-x^2/9 + y^2/4, x = -5 .. 5, y = -5 .. 5)`

Do you mean surface? Do you want the shortest path on the surface?

## correction...

@salim-barzani I believe you forgot to apply the chain rule in the part you did by hand. Here is a corrected version that verifies with pdetest on the original equation.

f12.mw

## Some notes...

@dharr I am trying to understand how you get from the pde to the ode. I can't get Maple to do this so I am wondering if there is some hidden assumption here that needs to be specified. If so, that may be why pdetest fails. Or the solution may not be correct.

pde-solve2.mw

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