Starting at 13 every other number is 5 more than the previous: 13, 18, 23 then 28, 33, 38, 43 and so on. The numbers in between are double their predecessors: 26, 36 then 46, 56, 66, 76, and so on. Therefore we can write the series: 13, 26, 18, 36, 23, 46, 28, 56, 33, 66, 38, 76, 43, 86, ...