what comes next 6,21,65,195,

Question

In number sequences, I want to need the answer and solution for the above mentioned question.And I want to need some links to study like this questions.

Answer 1

  

 

   6 =  5 x 1 + 1

 

  21 =  5 x 4 + 1

 

  65 =  5 x 13

 

  195 = 5 x 39

 

   

The multipliers of 5 is the gap sequence in the shell sort algorithm