# Series of inverse functions, unclear numerical constant

I was answering another question here and came up with this simple illustrative example that should have an analytic solution. Indeed it has, but I do not understand it. In particular, where 85 is coming from?

g[x_]:=BesselJ[0,x]
f[x_]:=Exp[x]
Series[f[InverseFunction[g][y]],{y,0,0}]
Out[1]= E^-BesselJZero[0,85]+O[y]^1

• I get E^-BesselJZero[0, 1] + SeriesData[y, 0, {}, 0, 1, 1], maybe try with a fresh kernel ? Oct 17 '20 at 8:43
• @b.gates.you.know.what That's a fresh kernel :) My \$Version is 11.1.1 for Mac OS X x86 (64-bit) (April 27, 2017) Oct 17 '20 at 9:05
• Mine is 12.1.1; I confirm that I get the same as you with 11.1.1. Oct 17 '20 at 9:17
• @b.gates.you.know.what Bug? Oct 17 '20 at 9:19
• I'd wait for others to confirm but you could already report it to Wolfram. Oct 17 '20 at 9:22