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## Saturday, May 21, 2011

### Equivalence classes in Mathematica

An equivalence relation like 'has the same remainder after division by 5' partitions a set in equivalence classes.

The relevant Mathematica function is GatherBy[ set, equivalence relation] -> {equivalence classes}:

In[5]:= GatherBy[{1, 2, 3, 4, 5}, Mod[#, 5] &]

Out[5]= {{1}, {2}, {3}, {4}, {5}}

In[6]:= GatherBy[{49, 33, 11, 1, 2, 3, 4, 5}, Mod[#, 5] &]

Out[6]= {{49, 4}, {33, 3}, {11, 1}, {2}, {5}}

Or when using the equivalence relation 'Is Odd?' on the same set:
In[7]:= GatherBy[{49, 33, 11, 1, 2, 3, 4, 5},OddQ]

Out[7]= {{49, 33, 11, 1, 3, 5}, {2, 4}}

## Welcome to The Bridge

Mathematics: is it the fabric of MEST?
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My continuous mission
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(Raumpatrouille – Die phantastischen Abenteuer des Raumschiffes Orion, colloquially aka Raumpatrouille Orion was the first German science fiction television series. Its seven episodes were broadcast by ARD beginning September 17, 1966. The series has since acquired cult status in Germany. Broadcast six years before Star Trek first aired in West Germany (in 1972), it became a huge success.)