effective Convexity error in formula?

CFA Level I
#1

Elan describes it as:

PV_ - PV+ - (2x PVo) / Change Y ^2 x PVo

Schweser describes the top line as

PV_ + PV+

So essentially schweser says you add the first two components, Elan says subtract. Is this an error on Elan’s part?

#2

From the CFA readings (v5- pg 501)- it’s [(V_) + (V+) -2(V0)]/[2(V0)(Dvsquared)]

#3

Schweser has it correct.

I wrote an article on convexity that may be of some help here: http://financialexamhelp123.com/convexity/.