Classic Fourier Series

01272015, 07:15 PM
Post: #1




Classic Fourier Series
hi,
I need something to calculate, in HP Prime, the nth Fourier coefficient (like in HP50: fourier(x^2,0) > 4π^2 / 3). I can't find it in catalog of Prime. There are only FFT and its inverse... help :) Salvo 

01302015, 05:20 PM
(This post was last modified: 01302015 05:37 PM by Snorre.)
Post: #2




RE: Classic Fourier Series
Hello,
why not define it by yourself: a(f,k):=int(f(t)*cos(k*t),t=π..π)/π b(f,k):=int(f(t)*sin(k*t),t=π..π)/π or something similiar. (Beware: this may suffer from a little issue which will be fixed in next software version.) Greetings 

01302015, 06:07 PM
Post: #3




RE: Classic Fourier Series
(01302015 05:20 PM)Snorre Wrote: Hello, thank you, I'll think about this suggestion... I hope also the little issue could solved soon However, it would be likely to have such a function included in the CAS, if it's possible ;) cheers Salvo 

01302015, 06:48 PM
(This post was last modified: 01302015 06:51 PM by salvomic.)
Post: #4




RE: Classic Fourier Series
define the functions it works, but I get first a lot of warnings, like those claiming the use of Taylor or subs() and so on...
ex.: "Warning, evalueting univariate expression(value) like if expression was a function. You should write subst(t^2,t,taylorx21" if I try with t^2 as function 

01302015, 07:23 PM
Post: #5




RE: Classic Fourier Series
(01302015 06:48 PM)salvomic Wrote: define the functions it works, but I get first a lot of warnings, like those claiming the use of Taylor or subs() and so on... This may have to do with using an expression as a function. Are you passing a CAS function as your argument? Or are you passing a CAS expression? Graph 3D  QPI  SolveSys 

01302015, 07:32 PM
(This post was last modified: 01302015 07:34 PM by salvomic.)
Post: #6




RE: Classic Fourier Series
(01302015 07:23 PM)Han Wrote: This may have to do with using an expression as a function. Are you passing a CAS function as your argument? Or are you passing a CAS expression? I tried this a(t^2,1) *** then some variables were a mess, until I purge some of them: d, g, v, used in another programs... 

01302015, 07:54 PM
Post: #7




RE: Classic Fourier Series
(01302015 07:32 PM)salvomic Wrote:(01302015 07:23 PM)Han Wrote: This may have to do with using an expression as a function. Are you passing a CAS function as your argument? Or are you passing a CAS expression? If you look at how a(f,k) was defined, the f is assumed to be a CAS function (notice the f(t) reference in how a(f,k) is defined). This means you need to pass a CAS function. f(t):=t^2; a(f,1); The CAS allows one to use an expression as a function. (This does not mean that it is good practice.) That is, if we define f as an expression f:=t^2, and then do f(2), it will not only give a warning about using functional notation to evaluate an expression, but also give you the result 4. Graph 3D  QPI  SolveSys 

01302015, 09:06 PM
Post: #8




RE: Classic Fourier Series
(01302015 07:54 PM)Han Wrote: If you look at how a(f,k) was defined, the f is assumed to be a CAS function (notice the f(t) reference in how a(f,k) is defined). This means you need to pass a CAS function. in this way it works (I used also variant with Int(...,0, 2π) ), but I tried this: create a0 (first term), ak, bk variable with expressions f(t):=sin(t) I got warning about the absolute value... then I got value for a0 (as a0:=(1/2π)Int(f(t),t,0,2π) After this value for a0, ak, bk and also f=f(t) are reset and I must rewrite all three functions... There is a reason? Is is possible to maintain the function Perhaps is better a little program that give a list with (a0, ak, bk)? 

01312015, 12:05 AM
(This post was last modified: 01312015 12:25 AM by Han.)
Post: #9




RE: Classic Fourier Series
(01302015 09:06 PM)salvomic Wrote:(01302015 07:54 PM)Han Wrote: If you look at how a(f,k) was defined, the f is assumed to be a CAS function (notice the f(t) reference in how a(f,k) is defined). This means you need to pass a CAS function. I'm not sure I understand what you did; perhaps you could copy/paste the actual commands you used? I am not having any trouble with respect to things "disappearing" Graph 3D  QPI  SolveSys 

01312015, 06:27 AM
Post: #10




RE: Classic Fourier Series
(01312015 12:05 AM)Han Wrote: I'm not sure I understand what you did; perhaps you could copy/paste the actual commands you used? I am not having any trouble with respect to things "disappearing" ok, I'll try to copy/paste as soon as possible later, however I tried to do like you did. The issue was after the message "warning"... Later I'll try again. For now I'm trying also with a program like this: Code:
it should give a0 (the first), ak and bk coefficient for k and a function. It could be made better; if I miss the local command for variables it give formulas with something like 256 or 1024 added, I don't understand why... Thank you for your effort! Salvo 

01312015, 09:11 PM
(This post was last modified: 01312015 09:17 PM by Snorre.)
Post: #11




RE: Classic Fourier Series
Hello Salvo,
a program is always a good idea ;) Just two tips: 1st: You should take care of the argument types your program expects. Your code seems to expect fun to be an expression (not a function) with t (and only t) being the independent variable, so the typical usage looks like: fourcoeff(t²,1) or f(x):=x² and fourcoeff(f(t),1). By that you have to always remember that fun must be an expression of t. And furthermore, t must not be defined already. If you change your program slightly by Code: ... 2nd: Your program returns a sequence (an object similiar to those created by the builtin seq function), not a vector/list. That is totally OK as long as it was your intention. Just be aware that sequences behave quite differently when used in further calculations, for example 0+fourcoeff(...) vs. 0+[fourcoeff(...)]. If you want your program to return a vector or list, put the last expression return a0,ak,bk; within brackets or braces: return [a0,ak,bk]; or return {a0,ak,bk};. Greetings 

01312015, 09:29 PM
(This post was last modified: 01312015 09:29 PM by Tim Wessman.)
Post: #12




RE: Classic Fourier Series
(01302015 06:07 PM)salvomic Wrote: However, it would be likely to have such a function included in the CAS, if it's possible ;) XCAS has 3 commands for this. When we was going through the list of commands at some point in the past to remove things that we did not have time to fully test or made no sense on the handheld calculator I suspect this were missed unintentionally. Those commands are fourier_an, fourier_bn, fourier_cn defined as: Code: Returns the nth Fourier coefficient an=2/T*integrate(f(x)*cos(2*pi*n*x/T),a,a+T). I assume those will do what you are looking for? To everyone else, *please* point out missing commands from xcas if you think there is something that should be in Prime that is missing. Note that each command that is added however introduces additional overhead in the form of testing, documentation and so forth. Simply "turning everything on" does not make sense because of time contraints, the large numbers of synonyms/intermingled french command names, things that do not have compatible/valid syntax on the HP side, or things that just can't be supported for one reason or another at the moment (like writing out a wav file... yes, the cas will do that with the "writewav" command...) TW Although I work for the HP calculator group, the views and opinions I post here are my own. 

01312015, 09:47 PM
(This post was last modified: 01312015 10:20 PM by salvomic.)
Post: #13




RE: Classic Fourier Series
(01312015 09:29 PM)Tim Wessman Wrote: XCAS has 3 commands for this. When we was going through the list of commands at some point in the past ... yes, Tim! I mean those commands, all three (cn is for the exponential series)... So, they will be no more in the CAS, never in the next update? Ok, then I must just my little program (see above), also if it would be better to have them integrated... For me there few things that I could see missed in confront of my "old" HP 50g: these ones and the command "image" that in the HP50g is in the section "Linear Application" (with mkisom, ker...), for a matrix... Another good thing to have would be "Directional Derivative" (see my post here) For the rest Prime has already almost everything (excluding Graph3D and QPI, for example) I'm adding only something to calculate inverse (pseudo inverse) of a matrix, and few other useful programs... thank you, Salvo 

01312015, 09:53 PM
(This post was last modified: 02012015 10:05 AM by salvomic.)
Post: #14




RE: Classic Fourier Series
(01312015 09:11 PM)Snorre Wrote: Hello Salvo, hi friend, your tips are very welcome! I'm changing my program in this way. I'm still a newcomer in this programming language, so I love to learn more and more tips! Yes, however, I would export a list of three arguments: a0 (however this one is always the same), ak, bk... Last: doing so: Code:
I set g:=x^2 then input fourcoeff(g(x),1) or fourcoeff(x^2,1) and I get "warning, evaluating invariable expression(value) like if expression was a function. You should write subst(x^2,x,t)"... Sure I miss something else I would better have the possibility to input both functions or expressions :) For the rest, I would also use the program (or the function we discussed above) with some "piecewise" functions. So I tried to define a function for "square wave", f(t):=PIECEWISE(π<t<0,1,t=0,0,t=π,0,t=π,0,0<t<π,1); this function seem to be ok, but if I use it with the program or with other integrate function I get "No case applies"... Is this the way to use "Piecewise" function or I miss something? Thank you! Salvo 

02012015, 03:10 AM
(This post was last modified: 02012015 03:51 AM by Han.)
Post: #15




RE: Classic Fourier Series
(01312015 09:53 PM)salvomic Wrote: Yes, however, I would export a list of three arguments: a0 (however this one is always the same), ak, bk... Again, you need to understand the difference between a function and an expression. g:=x^2; < this creates an expression whose name is g g(x):=x^2; < this creates a function whose name is g a0:=int(fun(t),t,0,2*PI)/(2*PI); < this creates a variable named a0 whose value is the result of integrating a function named 'fun' (which hopefully you've already predefined); if 'fun(t)' is not defined, then a0 is an expression (otherwise a0 is presumably a numerical value) ak:=int(fun(t)*cos(k*t),t,0,2*PI)/PI; < this creates a variable named ak whose value is the result of integrating the product of the function 'fun(t)' and 'cos(k*t)'; if 'fun' and 'k' were predefined prior to creating ak, then ak is just a numerical value; otherwise ak is an expression (same as a0) Neither a0 nor ak are functions when defined this way. However, the CAS will let you get away with using expressions as if they were functions  it will just always throw warnings/complaints about using expressions as if they were functions. So don't do it if you don't want to see the warnings. ak(f,k):=int(f(t)*cos(k*t),t,0,2*PI)/(2*PI); < this creates a function named ak, whose arguments are a CAS function f and a constant k; that means 'f' must be a function (see the g(x):=x^2 example), and NOT an expression (see the g:=x^2 example) Once you understand the difference between the two, your warnings will go away. Graph 3D  QPI  SolveSys 

02012015, 03:22 AM
Post: #16




RE: Classic Fourier Series
(02012015 03:10 AM)Han Wrote: Again, you need to understand the difference between a function and an expression. Han  thanks for these explanations/clarifications; your detailed examples are very useful for learing to use CAS, and some of the more exotic features. fyi  For your final example, in the explanation 'a0' should be 'ak', that is if I understood it correctly. Bob Prosperi 

02012015, 03:51 AM
Post: #17




RE: Classic Fourier Series
(02012015 03:22 AM)rprosperi Wrote: fyi  For your final example, in the explanation 'a0' should be 'ak', that is if I understood it correctly. You are quite right; I've gone back and made the proper edits. Thank you! Graph 3D  QPI  SolveSys 

02012015, 09:32 AM
Post: #18




RE: Classic Fourier Series
(02012015 03:10 AM)Han Wrote: ... thank you, you are very kind about this explanation, very clear, now, for me... I'll try some variations in the program. Have a nice Sunday! ∫aL√0mic (IT9CLU) :: HP Prime 50g 41CX 71b 42s 39s 35s 12C 15C  DM42, DM41X  WP34s Prime Soft. Lib 

02012015, 01:52 PM
(This post was last modified: 02012015 05:07 PM by salvomic.)
Post: #19




RE: Classic Fourier Series
(01312015 09:11 PM)Snorre Wrote: a program is always a good idea ;) Hi Snorre, after some tries now I'm working on this idea, just for fun (and to understand): Code:
With this, after having defined g(t):=t^2, I get: fourcoeff(g,1) > ok, correct value fourcoeff(t^2,1) > ok, correct value, without any warning fourcoeff(x^2,1) > exit with the message. This is a workaround, sure, and the the program could be strongly improved! So the "type" control is working (it now handle a function in t and and expression in t), but it would better to set another control to say "if tipo=DOM_SYMBOLIC and the letter is not "t" substitute any letter like x, y, z... into "t") or reformat fun in fun(t) in some way but a efficient one (now the expression could have "t" without to be correct)... Thank you for patience ∫aL√0mic (IT9CLU) :: HP Prime 50g 41CX 71b 42s 39s 35s 12C 15C  DM42, DM41X  WP34s Prime Soft. Lib 

02012015, 05:12 PM
(This post was last modified: 02012015 05:15 PM by Snorre.)
Post: #20




RE: Classic Fourier Series
Hi,
another approach: Code: #cas 

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