This is a simple division series; each number is one-half of the previous number.

In other terms to say, the number is divided by 2 successively to get the next result.

4/2 = 2
2/2 = 1
1/2 = 1/2
(1/2)/2 = 1/4
(1/4)/2 = 1/8 and so on.

This is a simple alternating addition and subtraction series. In the first pattern, 3 is added; in the second, 2 is subtracted.

This is an alternating number subtraction series. First, 2 is subtracted, then 4, then 2, and so on.

In this simple alternating subtraction and addition series; 1 is subtracted, then 2 is added, and so on.

In this series, each number is repeated, then 13 is subtracted to arrive at the next number.

In this alternating repetition series, the random number 21 is interpolated every other number into an otherwise simple addition series that increases by 2, beginning with the number 9.

This is a simple subtraction series. Each number is 6 less than the previous number.

This alternating addition series begins with 3; then 1 is added to give 4; then 3 is added to give 7; then 1 is added, and so on.

This is a simple addition series with a random number, 8, interpolated as every other number. In the series, 6 is added to each number except 8, to arrive at the next number.

This is a simple alternating subtraction series, which subtracts 2, then 5.