Add property: for detecting if a graph has a pair of twin vertices

[math1um]

def is_v_twin(g,v): #for each neighbor w of v, check if N[v]=N[w]
    Nv=g.neighbors(v)
    Nvp=Nv+[v]
    for w in Nv:
        Nw=g.neighbors(w)
        Nw.append(w)
        if Set(Nvp) == Set(Nw):
            return True
    return False  

def has_twin(g): #True if there are vertices v,w with N[v]=N[w]
    for v in g.vertices():
        if is_v_twin(g,v):
            return True
    return False

def is_twin_free(g): #True if there are no vertices v,w with N[v]=N[w]
    return not has_twin(g)

[math1um]

The above code is mysteriously "flattened out" by the github editor!!!!

[math1um]

trying again to have code properly indented

def is_v_twin(g,v): #for each neighbor w of v, check if N[v]=N[w] Nv=g.neighbors(v) Nvp=Nv+[v] for w in Nv: Nw=g.neighbors(w) Nw.append(w) if Set(Nvp) == Set(Nw): return True return False

def has_twin(g): #True if there are vertices v,w with N[v]=N[w] for v in g.vertices(): if is_v_twin(g,v): return True return False

def is_twin_free(g): #True if there are no vertices v,w with N[v]=N[w] return not has_twin(g)

[math1um]

bad luck!

[nvcleemp]

You need to use three ` marks, not just one.

#trying again to have code properly indented

def is_v_twin(g,v): #for each neighbor w of v, check if N[v]=N[w]
    Nv=g.neighbors(v)
    Nvp=Nv+[v]
    for w in Nv:
        Nw=g.neighbors(w)
        Nw.append(w)
        if Set(Nvp) == Set(Nw):
            return True
    return False  

def has_twin(g): #True if there are vertices v,w with N[v]=N[w]
    for v in g.vertices():
        if is_v_twin(g,v):
            return True
    return False

def is_twin_free(g): #True if there are no vertices v,w with N[v]=N[w]
    return not has_twin(g)

[math1um]

thanks. amazingly hitting the "code" button in the editor gives you a single quote mark - and its not enough to leave the code indented. so dumb!

On Thu, Jul 8, 2021 at 3:31 PM Nico Van Cleemput @.***> wrote:

You need to use three ` marks, not just one

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--

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Automated Conjecturing for Sage: https://nvcleemp.github.io/conjecturing/ http://nvcleemp.github.io/conjecturing/

Graph Brain Project: https://github.com/math1um/objects-invariants-properties

VCU Discrete Math Seminar: https://go.vcu.edu/discrete http://www.people.vcu.edu/~nobushaw/dms.html

Craig Larson, Professor Department of Mathematics and Applied Mathematics Virginia Commonwealth University

4106 Grace E. Harris Hall 1015 Floyd Avenue Richmond, VA 23284-2014

[nvcleemp]

Yes, a single quote mark is for formatting a single line of text with a code-like font like this. For multiple lines of code you need to use the triple quote mark.

[jaritaes99]

added to properties