21
21 = 3 * 7, F8 8th Fibonacci number
221 - 1 = 7 * 7 * 127 * 337
1021 - 1 = 3 * 3 * 3 * 37 * 43 * 239 * 1933 * 4649 * 10838689
20 - 21 - 22
0 - 1 - 2 - 3 - 4 - 5 - 6 - 7 - 8 - 9
Mathematics Encyclopedia
Hellenica World - Scientific Library
