From the given number, take the squares of each digit and add them to the actual number. From the given sequence: Consider 11 --- Square of 1 is 1. Now add 11+1+1 to get the next term i.e, 13. Same follows for all the remaining numbers in the series. Consider 81 --- 8 square is 64 and 1 square is 1. Now add 81+64+1 to get the next term i.e, 146.