1. Only ones digit.
2. You cannot repeat numbers, unless they are repeated in the date. eg. 01, 11, 2001. You can use four ones, but only four ones.
3. You can use any mathematical operation.
4. You can use any exponent.
The game was invented by the quite brilliant teacher of Mathematics, Mr Date, whilst he was working at Tanglin Trust School in the late 90s. He still uses it today to entertain his wonderful Year 10 Add Maths class at Jerudong International School.