1st term =  31+1 = 3+1 = 4
 2nd term =  32-1 = 9-1 = 8
 3rd term =  33+1 = 27+1 = 28
 4th term =  34-1 = 81-1 = 80
 5th term =  35+1 = 243+1 = 244
 6th term =  36-1 = 729-1 = 728
 7th term =  37+1 = 2187+1 = 2188
 8th term =  38-1 = 6561-1 = 6560
 9th term =  39+1 = 19683+1 = 19684
10th term = 310-1 = 59049-1 = 59048
11th term = 311+1 = 177147+1 = 177148
12th term = 312-1 = 531441-1 = 531440
13th term = 313+1 = 1594323+1 = 1594324
14th term = 314-1 = 4782969-1 = 4782968
15th term = 315+1 = 14348907+1 = 14348908
16th term = 316-1 = 43046721-1 = 43046720
17th term = 317+1 = 129140163+1 = 129140164
18th term = 318-1 = 387420489-1 = 387420488
19th term = 319+1 = 1162261467+1 = 1162261468
20th term = 320-1 = 3486784401-1 = 3486784400
...

Edwin