Sequence 1,8,9,4, (), 1 / 6 who knows what to put in brackets

Sequence 1,8,9,4, (), 1 / 6 who knows what to put in brackets

This is really hard to think about, but you can fill in 1
1=1^4;8=2^3;9=3^2;4=4^1;(1)=5^0;1/6=6^-1