# What is a “linear set”

I'm reading "L'hypothèse du continu" by Sierpinski. He mentions many time "ensembles linéaires" or "linear sets" without defining this notion. Does anyone know what is the definition a such a set ?

Here is a translation into english of 2 proposition he gave :

"There exist a linear set whose cardinality is that of the continuum and whose image by any continuous function is of measure zero".

"There exist a linear set whose cardinality is that of the continuum and whose homeomorphic images have the same measure".

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After checking some translations, I can safely say that by ensemble linéaire he means a subset of the continuum, i.e., of $\Bbb R$.

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A set is said to be a linear set if one of the following conditions is satisfied:

• it is empty;
• it contains single element;
• it satisfies the following condition:

There are unique elements called "first" and "last" such that every element except the last has a unique successor, and every element except the first has a unique predecessor.

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Your answer differs significantly from Brian M. Scott's. Are you sure you've paid due attention to the context in which the question arose? –  Lord_Farin Jun 6 '13 at 9:37
According to this definition, the set $[0,1]\cup(2,3]$ would not be a linear set. –  M.G Jun 6 '13 at 13:49