Is 1021 prime?
let's see. no 3, no 7 (knock out the 21, left with 10=no), no 11 (subtract 11, left with 101=no), no 13 (1021+39 is 1060=106=53=no), no 17 (knock off the 102, left with 1=no), check 19, knock the left down to 95 which equals 71, so no), check 23, add 69 is 1090=109=no. check 29, add it is 1050=105=no, check 31, subtract it is 990=99=no, that's it. 37 squared is already gonna be way too high. so yes it's prime