Typing an equation in matlab with greek letters

Hello,

I have a problem with the greek letters in the line below. How can i insert the greek letters and also multiply them with each other the same time?

rs=(kB*T)/(4*π*η*D0);

Thank you!

[gonzalo Mier]

As far as I know you cannot use special symbols in matlab. You have to write them as pi and eta

[dpb]

Indeed, not allowable, not just what you know... :)

variable-names has the rules.

It doesn't seem as though there really is any direct reason the character set couldn't be extended, but never seems to occur. Of course, there are potential issues with internationalization and not all character sets are the same, but doesn't seem insurmountable.

Answer 1

[Walter Roberson]

syms kB T D0
Pi = evalin(symengine, '`π`');
nu = evalin(symengine, '`η`');
rs=(kB*T)/(4*Pi*nu*D0);

rs =

(T*kB)/(4*D0*`η`*`π`)

... Just don't ask to symvar(rs)

[Panagiota Chatzi]

I have read in the tutorial that if you want to enter the Greek letter π you write \pi and for η you write \eta.

So i have (4*π*η*D0) Can it be like this ( 4*pi*eta*D0) or (4*\pi*\eta*D0) or (4*(\pi)*(\eta)*D0)?

[Walter Roberson]

You are looking at tex or latex information. If you only want to display predetermined equations then LiveScript permits inserting latex text as part of the equation but does not permit calculation with them and cannot display latex that is constructed at run time.

To put the symbols into something you can calculate with then you need to use the symbolic engine internal representation of special characters and you also need to fight against the symbolic toolbox thinking that symbols should be convertible to ASCII unless specially marked like I did.

[Panagiota Chatzi]

Thank you very much!

[Gary Gorman]

Walter, your code will not will not run on my system. MacOs bigSur 11.6.5, R2022b. The followin errors are displayed in the livescript window:

Error using symengine

Unable to evaluate to Boolean.

Error in sym/symvar (line 45)

v = mupadmex('symobj::symvar', S.s, num2str(n), scell{:});

Error in sym/getSuggestionsForSym (line 1552)

varsReqSym = setdiff(string(symvar(s)), evalin('base', "string(who)"));

Error in sym/display (line 37)

symSuggestions = getSuggestionsForSym(X,namestr);

[Walter Roberson]

@Gary Gorman

syms kB T D0
Pi = evalin(symengine, '`π`');
nu = evalin(symengine, '`η`');
rs=(kB*T)/(4*Pi*nu*D0);
disp(char(rs))

(T*kB)/(4*D0*`η`*`π`)

Asking to display rs by itself in Livescript for some reason now invokes symvar() internally; earlier I had said "... Just don't ask to symvar(rs)"

Anyhow, if you are using LiveScript these days you can instead use

syms Kb T D0 nu
Pi = sym(pi);
rs=(kB*T)/(4*Pi*nu*D0);
rs

rs = 

If you are using traditional scripts or functions you will instead see

(T*kB)/(4*D0*nu*pi)

[Gary Gorman]

Thank you very much, Walter. That does it perfectly.

I notice that the treatment of "Pi" and "nu" by the Matlab interpreter is different. "Pi" needs no declaration in the "syms" command. "nu" does require a declaration in the "syms" command.

For greek letters, is there list anywhere that identifies the greek letters which are native to the Matlab interpreter and which require declaration in the "syms" command? My guess is no as this feature space of Matlab seems to be evolving quickly.

Thank you very much for your help!!!

[Walter Roberson]

Pi = sym(pi);
Catalan = catalan;
Eulergamma = eulergamma;
syms alpha beta gamma delta epsilon zeta eta theta iota kappa lambda mu nu xi omicron rho sigma tau upsilon phi chi psi omega
syms Alpha Beta Gamma Delta Epsilon Zeta Eta Theta Iota Kappa Lambda Mu Nu Xi Omicron Rho Sigma Tau Upsilon Phi Chi Psi Omega

[Pi Catalan Eulergamma]

ans = 

[alpha beta gamma delta epsilon zeta eta theta iota kappa lambda mu nu xi omicron rho sigma tau upsilon phi chi psi omega]

ans = 

[Alpha Beta Gamma Delta Epsilon Zeta Eta Theta Iota Kappa Lambda Mu Nu Xi Omicron Rho Sigma Tau Upsilon Phi Chi Psi Omega]

ans = 

With respect to π you can do

syms pi
pi

pi = 

π

and you will see a symbol being output. But...

vpa(Pi)    %the one using sym(pi) -- gives numeric approximation

ans = 

3.1415926535897932384626433832795

vpa(pi)    %the one using sym -- is just a symbol without a value

ans = 

π

That is, the rule these days is that if you syms a name, then the name becomes an unresolved symbolic variable with no specific properties. syms pi is just a symbolic variable with a name that happens to print out fancy, same as syms alpha . But if you want the symbol to stand in for the irrational constant then you need to use sym()

The default conversion for sym() is the r ("rational") conversion, which takes the input number and first checks to see if it is close to a rational ratio of π with denominator no greater than 1000; if so then the conversion is to the rational. If not then sym() looks to see if the value is sufficiently close to square root of an rational; if so then a ^ term is emitted. If not then sym does some kind of continued fraction analysis to see if the input is close to a rational; if so the rational is emitted. If none of these hold then the rational implicitly coded in the binary floating point number is used; binary floating point codes numbers as an integer divided by 2^53, times 2 to some power, and sym() will create the integers and build an appropriate rational.

These behaviours are hard-coded in sym()

sym(1/3 - 33*eps)

ans = 

sym(1/3 - 34*eps)

ans = 

So when you sym(pi) it is not looking at the name of the function being passed in; it is looking at the value and finding it is close enough to the irrational constant and is encoding in terms of the transcendent constant.

[Walter Roberson]

There are a few names with variant forms available. As it is not immediately obvious how a variant form of a capital would be expressed, let us try the possibilities:

syms varalpha varbeta vargamma vardelta varepsilon varzeta vareta vartheta variota varkappa varlambda varmu varnu varxi varomicron varrho varsigma vartau varupsilon varphi varchi varpsi varomega
syms varAlpha varBeta varGamma varDelta varEpsilon varZeta varEta varTheta varIota varKappa varLambda varMu varNu varXi varOmicron varRho varSigma varTau varUpsilon varPhi varChi varPsi varOmega
syms Varalpha Varbeta Vargamma Vardelta Varepsilon Varzeta Vareta Vartheta Variota Varkappa Varlambda Varmu Varnu Varxi Varomicron Varrho Varsigma Vartau Varupsilon Varphi Varchi Varpsi Varomega
syms VarAlpha VarBeta VarGamma VarDelta VarEpsilon VarZeta VarEta VarTheta VarIota VarKappa VarLambda VarMu VarNu VarXi VarOmicron VarRho VarSigma VarTau VarUpsilon VarPhi VarChi VarPsi VarOmega

[varalpha varbeta vargamma vardelta varepsilon varzeta vareta vartheta variota varkappa varlambda varmu varnu varxi varomicron varrho varsigma vartau varupsilon varphi varchi varpsi varomega]

ans = 

[varAlpha varBeta varGamma varDelta varEpsilon varZeta varEta varTheta varIota varKappa varLambda varMu varNu varXi varOmicron varRho varSigma varTau varUpsilon varPhi varChi varPsi varOmega]

ans = 

[Varalpha Varbeta Vargamma Vardelta Varepsilon Varzeta Vareta Vartheta Variota Varkappa Varlambda Varmu Varnu Varxi Varomicron Varrho Varsigma Vartau Varupsilon Varphi Varchi Varpsi Varomega]

ans = 

[VarAlpha VarBeta VarGamma VarDelta VarEpsilon VarZeta VarEta VarTheta VarIota VarKappa VarLambda VarMu VarNu VarXi VarOmicron VarRho VarSigma VarTau VarUpsilon VarPhi VarChi VarPsi VarOmega]

ans = 

The answer seems to be that there are no variant capitalized forms.

To summarize the available variant forms:

[varepsilon vartheta varsigma varphi]

ans = 

[Walter Roberson]

Note: above I showed catalan and eulergamma because sym() specifically documents both of them as losing their special meaning as constants if you syms them. They do not happen to have a special display symbol (at least not at this time... I think I have seen the gamma symbol being output from a symsum() of an appropriate infinite series.)

[Gary Gorman]

Thank for that very comprehensive answer, Walter. I think I understand it. I deduce the following:

1) Approximately 23 greek letter symbols, both lower case ans upper case, can be displayed in the livescript output pane if the syms command is used to declare them.

2) Anything declared with syms does not have an underlying value associated with it.

3) All distinct character strings declared on a syms command, but not yet used in assignment statement, have identical properties EXCEPT for the rendered value in the livescript output pane. Livescript output pane rendering is done WYSIWYG with the exception of the 46 greek letters (23 lower case and 23 upper case) which are rendered as shown above.

I dont care if its 23 or 46 or whatever. I am just trying to see if I am close to understanding the rendering engine for single variables in the livescript window. Hopefully I have a good idea:) I have many other livescript output rendering confusions, but I want to proceed with a clear understanding of the single variable rendering rules. Thanks again for your help.

[Walter Roberson]

You forgot the four var* symbols ;)

[Walter Roberson]

https://www.mathworks.com/help/symbolic/add-subscripts-superscripts-accents-to-symbolic-variables.html is also important for this purpose

[Mohammad Qaza]

any idea how I can write two consecutive greek letters like Delta theta without having theta as subscript?

[Walter Roberson]

@Mohammad Qaza

That is a good question. I cannot think of any way to do it.