Let In=∫0π/4 tannxdx, then I2+I4,I3+I5,I4+I6,I5+I7,… are in
HP
GP
AP
AGP
In+In-2=1n−1
In+2+In=1n+1
I2+I4=13I3+I5=14I4+I6=15
13,14,15... are H.P