They are 'square' numbers. 6x6=36, 9x9=81 and 5x5=25. Some other square numbers are 64, 4, 9, 100 & 121
25 36 49 64 81
4, 9, 16, 25, 36, 49, 64, 81
They are all perfect squares.
1, 4, 9, 16, 25, 36, 49, 64, 81, 100
1, 4, 9, 16, 25, 36, 49, 64, 81
1, 4, 9, 16, 25, 36, 49, 64, 81, 100
1, 4, 9, 16, 25, 36, 49, 64, 81, 100
0, 1, 4, 9, 16, 25, 36, 49, 64, 81
1491625364964811001, 4, 9, 16, 25, 36, 49, 64, 81, and 100
1, 4, 9, 16, 25, 36, 49, 64, 81, 100, 121, 144
4,9,16,25,36,49,64,81,100,121
1; 4; 9; 16; 25; 36; 49; 64; 81; 100