Tools¶

sage.combinat.tools.transitive_ideal(f, x)

Return a list of all elements reachable from $$x$$ in the abstract reduction system whose reduction relation is given by the function $$f$$.

In more elementary terms: If $$S$$ is a set, and $$f$$ is a function sending every element of $$S$$ to a list of elements of $$S$$, then we can define a digraph on the vertex set $$S$$ by drawing an edge from $$s$$ to $$t$$ for every $$s \in S$$ and every $$t \in f(s)$$. If $$x \in S$$, then an element $$y \in S$$ is said to be reachable from $$x$$ if there is a path $$x \to y$$ in this graph. Given $$f$$ and $$x$$, this method computes the list of all elements of $$S$$ reachable from $$x$$.

Note that if there are infinitely many such elements, then this method will never halt.

EXAMPLES:

sage: f = lambda x: [x-1] if x > 0 else []
sage: sage.combinat.tools.transitive_ideal(f, 10)
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

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