18 is arithmatic progression but 19 is neither ap or gp. its actually HP
harmonic progression.
A sequence is said to be a harmonic progression (HP) , if the inverse of the sequence follow the rules of the arithmetic progression (AP).
In arithmetic progression, the common difference between any two consecutive terms always remains constant.
The inverse of the 2nd sequence contains the terms 2, 3, 4 and 5. The common difference between any two consecutive terms in this sequence is 1. so its HP