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1^1 undefined

asked 2018-11-13 02:58:44 -0600

Jsevillamol gravatar image

updated 2018-11-13 03:03:40 -0600

Consider the following code snippet:

def my_mul(x,y):
    if x == None: return y
    if y == None: return x
    return x*y

# Squarefree Decomposition in F_q[x]
def squarefree_decomposition(GF, f):
    p = GF.characteristic()
    # base case
    if f == 1: return [1]

    # recursive case
    df = f.diff()
    i_factors = [1]
    g = gcd(f,df)
    w = GF(f / g) 
    # collect squarefree factors such that p !| i
    while(w != 1):
        w_aux = gcd(g,w)
        g = GF(g / w_aux)
        i_factor = GF(w / w_aux)
        w = w_aux
    # here, g = prod[f_(j)^j for p | j]
    # collect squarefree factors such that p | j
    g_root = GF(g.coefficients(sparse=False)[::p]) # pth root of g
    g_root_decomposition = squarefree_decomposition(GF, g_root)
    j_factors = [g_root_decomposition[i/p] if i%p==0 else 1 for i in xrange(p*(len(g_root_decomposition)-1)+1)]

    # combine results
    res = map(my_mul, i_factors, j_factors)

    return res

# Example in F_q[x]
GF125X = GF(5^3)[x]
f = GF125X((x^5 + x^2 + x^1 + 1)^2*x^5)
decom = squarefree_decomposition(GF125X, f)
print("{} decomposes as {}".format(f, decom))
recom = [f_i^i for f_i,i in enumerate(decom)]

When I execute this, I get the following output:

x^15 + 2*x^12 + 2*x^11 + 2*x^10 + x^9 + 2*x^8 + 3*x^7 + 2*x^6 + x^5 decomposes as [1, 1, x^5 + x^2 + x + 1, 1, 1, x]
Error in lines 28-28
Traceback (most recent call last):
  File "/cocalc/lib/python2.7/site-packages/smc_sagews/", line 1188, in execute
    flags=compile_flags) in namespace, locals
  File "", line 1, in <module>
  File "sage/rings/polynomial/polynomial_template.pxi", line 605, in sage.rings.polynomial.polynomial_zz_pex.Polynomial_template.__pow__ (build/cythonized/sage/rings/polynomial/polynomial_zz_pex.cpp:12318)
    raise NotImplementedError("%s^%s not defined."%(ee,self))
NotImplementedError: 1^1 not defined.

I notice I am confused. AFAIK, there is only one sensible result for 1^1, namely 1. Why do I get this error?

If I try creating a 1 polynomial in a finite field and taking its 1th power it works:

id_ = GF125X(1)

Produces as output 1, as expected.

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1 answer

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answered 2018-11-13 04:55:21 -0600

updated 2018-11-13 05:12:09 -0600

The problem can be seen as follows:


Indeed it does not make sense to raise integers to polynomial powers in general.

Maybe the error message should be more descriptive, by including the types.

In your case, it seems you have confused f_i and i: note enumerate yields a list of pairs where the first element is the index.

Also you can omit the argument "GF" (which, confusingly, is a polynomial ring): it can be obtained from f by f.parent(). Furthermore you can probably avoid using my_mul by using prod and zip. Also, you will probably find it convenient to define

GF125X.<x> = GF(5^3)[]
f = (x^5 + x^2 + x^1 + 1)^2*x^5

so that x is really the generator of a polynomial ring, and you can define f without explicit conversion.

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Asked: 2018-11-13 02:58:44 -0600

Seen: 74 times

Last updated: Nov 13