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Lifts of circle-loop concatenations and reversals
Statement
Let and be based loops in , and let their lifts from zero be and , with terminal values and . The lift of from zero is
and it ends at . The lift of the reversed loop from zero is
and it ends at . Thus lifts of circle-loop concatenations and reversals have endpoints equal to the sum and the negative of the original endpoints.
Facts & Assumptions
Given: Based loops , their lifts from zero, and terminal values and .
For a based circle loop with lift from zero, the terminal value is an integer and (The degree of a based circle loop).
The product traverses first and second, and is represented by (Based loops and the fundamental group).
A path through a covering has a unique lift once its initial point is prescribed (Existence and uniqueness of path lifts through a covering map).
Functions continuous on each member of a finite closed cover, and agreeing where the pieces meet, paste to a continuous function (Continuity may be checked on any open cover, and on any finite closed cover; composites of continuous maps are continuous).
For the quotient projection , one has for every real and integer (The circle as with basepoint ).
Constant functions, the identity, finite sums, and scalar multiples are continuous on real intervals (Sums, scalar multiples, products, absolute values, maxima, minima and quotients with nonvanishing denominator of continuous functions are continuous, as are constants, the identity and every polynomial function).
Proof
Define by the displayed two-piece formula. At the left value is and the right value is , so [L4] and [L6] make continuous. It starts at zero. By [L5], its first half projects to and its second half to , in the order fixed by [L2], so ; its endpoint is .
Define . It is continuous by [L6], begins at , and ends at . Since by [L1], [L5] gives .
Both and are lifts with initial point zero, so uniqueness in [L3] identifies them with the defining lifts of and . Their endpoints are therefore and , respectively.
Depends on
- The degree of a based circle loop
- The circle as $S^1=\mathbb R/\mathbb Z$ with basepoint $[0]$
- Based loops and the fundamental group
- Existence and uniqueness of path lifts through a covering map
- Sums, scalar multiples, products, absolute values, maxima, minima and quotients with nonvanishing denominator of continuous functions are continuous, as are constants, the identity and every polynomial function
- Continuity may be checked on any open cover, and on any finite closed cover; composites of continuous maps are continuous
Used by
Dependency tree · next 3 levels
Direct dependencies and their dependencies through the next three levels: 106 results over 18 levels. An arrow runs from a result to what uses it, and this result sits at the bottom with a heavier outline. Click the chart to enlarge it.
Sources
- J. Peter May, A Concise Course in Algebraic Topology, Ch. 1, Section 5 (standard reference, not scraped)