# UndefVarError: \`𝕊ᶻ\` not defined

**URL:** https://discourse.julialang.org/t/undefvarerror-not-defined/124380
**Category:** General Usage
**Created:** [January 2, 2025, 5:03pm UTC](https://discourse.julialang.org/t/undefvarerror-not-defined/124380 "2025-01-02T17:03:34Z")
**Posts on this page:** 3
**Page:** 1

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### Author: ![JYOTI\_BISHT](https://avatars.discourse-cdn.com/v4/letter/j/8baadc/32.png) [@JYOTI\_BISHT](https://discourse.julialang.org/u/JYOTI_BISHT)
#### Post date: [January 2, 2025, 5:03pm UTC](https://discourse.julialang.org/t/undefvarerror-not-defined/124380/1 "2025-01-02T17:03:34Z")

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Hi Julia team,

I am trying this trial code given on julia website to do exact diagonalization using QuantumLattices. The problem is: writing the following code it says

UndefVarError: `𝕊ᶻ` not defined

using QuantumLattices  
using ExactDiagonalization  
using LinearAlgebra: eigen

# define the unitcell of the square lattice

unitcell = Lattice([0.0, 0.0]; name=:Square, vectors=[[1.0, 0.0], [0.0, 1.0]])

# define a finite 4×4 cluster of the square lattice with open boundary condition

lattice = Lattice(unitcell, (4, 4))

# define the Hilbert space (spin-1/2)

hilbert = Hilbert(Spin{1//2}(), length(lattice))

# define the quantum number of the sub-Hilbert space in which the computation to be carried out

# for the ground state, Sz=0

quantumnumber = 𝕊ᶻ(0) …

what could be the reason??

I am using Julia version 1.9.0. Is this symbol not supported or something??

Thank you in advance !!

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### Author: ![BioTurboNick](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/bioturbonick/32/6380_2.png) [@BioTurboNick](https://discourse.julialang.org/u/BioTurboNick)
#### Post date: [January 2, 2025, 5:13pm UTC](https://discourse.julialang.org/t/undefvarerror-not-defined/124380/2 "2025-01-02T17:13:21Z")

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Searching, it seems you’re referring to this package, not something related to Julia base: [Antiferromagnetic Heisenberg Model on square lattice · ExactDiagonalization.jl](https://quantum-many-body.github.io/ExactDiagonalization.jl/dev/examples/HeisenbergModel/)

And yeah it suggests that symbol is not exported from one of the packages. Perhaps the example is wrong or out of date.

For future reference, code examples are best pasted between sets of three backticks (```)

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### Author: ![JYOTI\_BISHT](https://avatars.discourse-cdn.com/v4/letter/j/8baadc/32.png) [@JYOTI\_BISHT](https://discourse.julialang.org/u/JYOTI_BISHT)
#### Post date: [January 2, 2025, 5:31pm UTC](https://discourse.julialang.org/t/undefvarerror-not-defined/124380/3 "2025-01-02T17:31:38Z")

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Okay thank you. I will keep this in mind from the next time “For future reference, code examples are best pasted between sets of three backticks (```)”.