# Solving an equation involving an integral

How do I get the following into Mathematica, solving for $a$:

$$0.7 = 1 - \frac{2}{a} \times \left[ \frac{1}{a} \int_0^a \frac{x}{\exp(x)-1}\mathrm dx + \frac{a}{6} - 1\right]$$

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• In particular, it is helpful to let us know what you have already tried? This site does not provide tutorial services. I would suggest first trying to evaluate Integrate[(x/(Exp[x] - 1)), {x, 0, a}] and see what that answer suggests to you. You can then use NSolve or maybe FindRoot to work out the rest. – Verbeia Nov 30 '15 at 6:03
• Thanks Verbeia. Can you tell me what the following won't return the Reals result and gives me back an equation?: 'NSolve[1 - (2/ a)*((1/a)*Integrate[(x/(Exp[x] - 1)), {x, 0, a}] + (a/6) - 1) == 0.7, a, Reals]' – Matt Nov 30 '15 at 6:27

In my experience FindRoot works best for such problems:

In:= fun[a_?NumericQ] := NIntegrate[(x/(Exp[x] - 1)), {x, 0, a}]

In:= FindRoot[1 - (2/a)*((1/a)*fun[a] + (a/6) - 1) == 0.7, {a, 0.1}]

Out= {a -> 58.3073}


One can also use the event detection capability (WhenEvent[]) of NDSolve[] to find the desired root:

(* (Exp[x] - 1)/x; see http://books.google.com/books?id=7J52J4GrsJkC&pg=PA20 *)
exm1x[x_?NumberQ] := With[{t = Exp[x], one = N[1, Precision[x]]},
Piecewise[{{one, t == 1}}, (t - 1)/Log[t]]]

NDSolveValue[{y'[x] == 1/exm1x[x], y == 0,
WhenEvent[7 x^2/10 == x^2 - 2 (y[x] + x^2/6 - x), "StopIntegration",
"DetectionMethod" -> Interpolation,
"LocationMethod" -> "Brent"]},
y["Domain"], {x, 0, ∞}, Method -> "Extrapolation",
AccuracyGoal -> 20, WorkingPrecision -> 20][[1, -1]]
58.307312765209898828


The integral has an exact representation, so it's possible to use several methods.

fun[a_] = Integrate[(x/(Exp[x] - 1)), {x, 0, a}, Assumptions -> a > 0]
(*  -(a^2/2) + I a π - π^2/6 + a Log[-1 + E^a] + PolyLog[2, E^a]  *)

FindRoot[1 - (2/a)*((1/a)*fun[a] + (a/6) - 1) == 7/10, {a, 1},
WorkingPrecision -> 20]
(*  {a -> 58.307312765239769644}  *)

NSolve[1 - (2/a)*((1/a)*fun[a] + (a/6) - 1) == 7/10 && 50 < a < 60, a,
WorkingPrecision -> 20]
(*  {{a -> 58.307312765239769644}}  *)


Of course, to use NSolve you need to give it a compact domain, so one needs some knowledge of where the root is.

Or if you want an exact representation,

sol = Root[{
1 - (2/a)*((1/a)*fun[a] + (a/6) - 1) == 7/10 /.
Equal -> Subtract /. a -> # // Function[bdy, bdy &],
a /. FindRoot[1 - (2/a)*((1/a)*fun[a] + (a/6) - 1) == 7/10, {a, 1},
WorkingPrecision -> 20]
}] N[sol, 50]
(*  58.307312765239769643612756575305892317507739080842 + 0.*10^-49 I  *)

• I'm seeing a number of questions involving Debye-like functions lately. I wonder why... – J. M.'s discontentment Feb 18 '16 at 3:11
• @J.M. I have no idea. This is a somewhat older question, though. – Michael E2 Feb 18 '16 at 3:25