In 1958 Mazur gave a remarkable telescoping argument to show that a smooth 3-sphere in 4-space bounds a topological 4-ball. We discuss some applications of his argument.
Links:
[1] https://www.mpim-bonn.mpg.de/taxonomy/term/39
[2] https://www.mpim-bonn.mpg.de/node/3444
[3] https://www.mpim-bonn.mpg.de/node/6656