Tham khảo Hình_tròn

  1. 1 2 The Concise Oxford Dictionary of Mathematics, 2014, ISBN 9780199679591 
  2. ↑ Intuitive Concepts in Elementary Topology, 2013, ISBN 9780486275765 .
  3. ↑ Journey into Mathematics: An Introduction to Proofs, 2013, ISBN 9780486151687 .
  4. ↑ Altmann, Simon L. (1992). Icons and Symmetries (bằng tiếng Anh). Oxford University Press. ISBN 9780198555995. 
  5. ↑ New Foundations for Physical Geometry: The Theory of Linear Structures, 2014, ISBN 9780191004551 .
  6. ↑ Combinatorial Group Theory: A Topological Approach, 1989, ISBN 9780521349369 .
  7. ↑ In higher dimensions, the Euler characteristic of a closed ball remains equal to +1, but the Euler characteristic of an open ball is +1 for even-dimensional balls and −1 for odd-dimensional balls. See Introduction to Geometric Probability, 1997 .
  8. ↑ Arnold (2013), p. 132.
  9. ↑ Arnold (2013), Ex. 1, p. 135.