Typing an equation in matlab with greek letters

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Panagiota Chatzi 2019-5-20

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评论： Walter Roberson 2024-9-8

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!

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gonzalo Mier 2019-5-20

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

dpb 2019-5-21

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.

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Answer 1

Walter Roberson 2019-5-21

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编辑：Walter Roberson 2019-5-21

在 MATLAB Online 中打开

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)

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Walter Roberson 2022-12-28

在 MATLAB Online 中打开

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 2022-12-28

在 MATLAB Online 中打开

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 =

Mohammad Qaza 2024-9-8

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

Walter Roberson 2024-9-8

@Mohammad Qaza

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

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