What is the rule for 1 2 4?

Answer 1

It suggests that it is '2' to a given power/index number/ exponential.

1 = 2^(0)

2 = 2^(1)

4 = 2^(2)

Then

2^(3) = 8

2^(4) = 16

et seq.

When the power is less than '0'

2^(-1) = 1/2^(1) = 1/2 ( 0.5)

2^(-2) = 1 / 2^(2) = 1/4 (0.25)

2^(-3) = 1/2^(3) = 1/8 = ( 0.125)

et seq.

Answer 2

The sequence is too short for a definitive answer.

It could be:+1, +2, +3 ...

Or it could be: *2, *2, *2