Average distance of two points in unit n-dimensional hypercube

An easy way to understand the unit n-dimensional hypercube is to consider the region of space in n dimensions that you can get if every coordinate component lies in [0, 1]. So for one dimension it's the line segment from 0 to 1, for two dimensions it's the square with corners (0, 0) and (1, 1), etc.

Write a program or function that given n returns the average Euclidean distance of two points uniformly random selected from the unit n-dimension hypercube. Your answer must be within 10-6 of the actual value. It's ok if your answer overflows your language's native floating point type for big n.

Randomly selecting a 'big' number of points and calculating the average does not guarantee such accuracy.

Examples:

1 → 0.3333333333...
2 → 0.5214054331...
3 → 0.6617071822...
4 → 0.7776656535...
5 → 0.8785309152...
6 → 0.9689420830...
7 → 1.0515838734...
8 → 1.1281653402...

Data acquired from MathWorld.

This is , lowest byte-count wins.

• Reiterated winning criterion in an edit. – Magic Octopus Urn Feb 1 '17 at 20:54
• Just to be clear: distance refers to the Euclidean distance, yes? – Dennis Feb 1 '17 at 20:56
• @carusocomputing What's the point of the challenge if you want me to solve it for you? – orlp Feb 1 '17 at 21:39
• @orlp registering my objection to a challenge that is a math puzzle until someone figures out the math, then it becomes a programming puzzle when everyone copies the formula in different languages. I need to ask a meta question about this. – Sparr Feb 1 '17 at 22:22
• When you say 5 digits of accuracy, do you mean to within 1e-5, or would an estimate of 1.500000000000001 be wrong when the output should be 1.499999999999999? – xnor Feb 1 '17 at 22:49

Mathematica, 68 bytes

NIntegrate[(1-((E^-u^2+u*Erf@u√π-1)/u^2)^#)/u^2,{u,0,∞}]/√π&


Implementation of the formula using NIntegrate to approximate its value.

• Beat me by 58 seconds :( – JungHwan Min Feb 1 '17 at 23:30
• Where did you find the formula? – feersum Feb 2 '17 at 22:58
• @feersum It's formula (8) from Hypercube Line Picking. (10) is a typo and shouldn't have the double integral or du. – miles Feb 2 '17 at 23:03
• If you actually input this as text it parses u√π as one token, so you need a space between u and √. – feersum Feb 2 '17 at 23:41