Abacaba Number

An abacaba number is the number of letters in an Abacaba sequence where the middle letter is represented by a number from 1-26. It is represented by Ax where x is the value of the middle number.

Formula
An = 2(An-1)+1

List of Abacaba numbers

 * A1 = 1
 * A2 = 3
 * A3 = 7
 * A4 = 15
 * A5 = 31