Igor Schein on Mon, 15 May 2006 20:52:10 +0200

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need for a hash function


Here's a scenario.  I need to to check whether or not 2 polynomials
are isomorphic.  The polynomials are big, so the usual methods like
nfisisom() and friends are computationally costly.  But, I am able to
compute relatively cheaply my own invariant and compare it for both
polynomials.  However, the value of the function which computes
the invariant takes on huge vectors.  I can still compare the value,
but output to the screen is impossible.  If F is my invariant, here's
a trick I employ (could be bettered, I am sure, but I don't feel like
playing with escaping special characters any longer than it took me to
come up with the first draft):

Hash(obj)=extern("echo '"obj"'|md5sum|cut -d\\  -f1|sed 's/^/_/'|sed 's/$/_/'|tr _ '\\042'")

for example:



? Hash(F(polzagier(100,98)))

Nice and compact.  Now if I replace F with the actual function, it
enables me to compare polynomials not only logically, but also
visually, e.g. if I have a vector V of such polynomials, I would do

for(k=1,#V,print(k," ",Hash(F(V[k]))))

Did I make a strong enough case for the need of internal hashing
function?  Can anyone think of other scenarions which would make
adding such function more attractive?