# Error in using EventLocator in NDSolve

I have a problem in using EventLocator in Mathematica 9. However, this error doesn't show up in Mathematica 8 on my labtop.

My code is,

η=G[x[n],y[n]];

pde = {x''[n] == -(1 - (x'[n]^2 + y'[n]^2)/(4*x[n]^2))*(3*x'[n] +
2*x[n]^2*\!$$\*SubscriptBox[\(\[PartialD]$$, $$x[n]$$]V\)/V) + (x'[n]^2 - y'[n]^2)/
x[n], y''[n] == -(1 - (x'[n]^2 + y'[n]^2)/(4*x[n]^2))*(3*y'[n] +
2*x[n]^2*\!$$\*SubscriptBox[\(\[PartialD]$$, $$y[n]$$]V\)/V) +
2*(x'[n]*y'[n])/x[n]};
bc = {x[0] == saddle, y[0] == 0.01, x'[0] == 0, y'[0] == 0};
Sol = NDSolve[{pde, bc}, {x, y}, {n, 0, 1000}, MaxSteps -> 50000,
AccuracyGoal -> Automatic,
Method -> {"EventLocator", "Event" -> η - 1,
"EventCondition" -> (n > 60)}]


The error message is,

NDSolve::evboo: The value of the event function at n = 61.76853457162405 was not True or False. When not True, the value will be considered as False. >>

This is quite strange as there is no problem in running this on Mathematica 8.

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Thanks for editing it in a nice format. How can I post my codes in that way? (It is my first time uploading codes) – mastrok Jan 13 '14 at 8:47
mastrok, welcome to the site, and thank you for caring about the formatting of your posts. On every edit box there is a small yellow circle with white "?" in it; click that to bring up an editing help bar. Clicking "Code" on that bar will bring up a guide for formatting code. Also see mathematica.stackexchange.com/editing-help (The same as the "advanced help" link on that bar.) More directly there are a series of buttons above the editing box; fifth in from the left is a "{ }" button which will format a selection as code, or you can just press Ctrl+K. – Mr.Wizard Jan 13 '14 at 8:53
Er… where's the definition of V and G and saddle? – xzczd Jan 14 '14 at 3:03
V and G are smooth function of x and y (their expressions are quite long). I was trying to solve the equations of motions of a dynamical system. V can be regarded as some potential while G is a function calculating a geometrical property of V. saddle is the saddle point of V. This code works on my laptop (mathematica 8) but fails on my desktop (version 9) – mastrok Jan 14 '14 at 6:11
Without a complete code sample, it'll be hard for others to figure out the reason… If the original definitions of V and G are really long, you can provide a simplified version reproducing your problem. BTW, if you add @xzczd` at the beginning of the comment, I'll get a remind. – xzczd Feb 4 '14 at 6:04