What is the integral of #1/ln(lnx)#?

1 Answer
Jun 29, 2015

This is an impossible integral to complete without resorting to calculators and evaluating at explicit bounds.

http://www.wolframalpha.com/input/?i=integral+of+1%2F%28ln%28lnx%29%29

Let's see how far we can go, though...

#int (ln(lnx))^(-1)dx#

Let:
#u = lnx#
#du = 1/xdx#
#xdu = dx -> e^udu = dx#

#= int (lnu)^(-1)e^udu#

#= inte^u/(lnu)du#

#st - int t ds#

Let:
#s = e^u#
#ds = e^udu#
#dt = 1/lnudu#
#t = ?#

Detour. We need to know the integral for #1/lnx#. Let:
#q = 1/lnu#
#dq = -lnu/udu#
#dr = du#
#r = u#

#qr - int r dq#

#= u/lnu + int lnudu#

#= u/lnu + u ln|u| - u = t#

Back to #s# and #t#:

#= e^u (u/lnu + u ln|u| - u) - int e^u(u/lnu + u lnu - u)du#

#= (ue^u)/lnu + ue^u ln|u| - u e^u - int (ue^u)/lnudu - int ue^u lnudu + intu e^udu#

The only integral we can do with real functions or standard functions is #int u e^udu#. I'm running out of variables.

#op - int p do#

Let:
#o = u#
#do = du#
#dp = e^udu#
#p = e^u#

#ue^u - int e^udu#

#= ue^u - e^u#

So now we get:

#= (ue^u)/lnu + ue^u ln|u| cancel(- u e^u + ue^u) - e^u - int (ue^u)/lnudu - int ue^u lnudu#

#= (ue^u)/lnu + ue^u ln|u| - e^u - int (ue^u)/lnudu - int ue^u lnudu#

#= e^u[u/lnu + u ln|u| - 1] - int (ue^u)/lnudu - int ue^u lnudu#

#= e^(lnx)[(lnx)/ln(lnx) + (lnx) ln|lnx| - 1] - int ((lnx)e^(lnx))/(xln(lnx))dx - int 1/x(lnx)e^(lnx) ln(lnx)dx#

#= color(blue)(x[(lnx)/ln(lnx) + (lnx) ln|lnx| - 1] - int (lnx)/(ln(lnx))dx - int (lnx) ln(lnx)dx)#