!# $I_n(u) = n \int_{t=0}^u t^n dt/(e^t-1)$ for $n = 2$ and $n = 3$.
!#  
!# 1st line: number of values of $u$.
!#  
!# Following lines:
!#   1st column: $u$.
!#   2nd column: $I_2(u)$.
!#   3rd column: $I_3(u)$.
!#  
!# Function $f_n(x)$ of Li & Draine 2001 (ApJ 554, 778) is computed as 
!#
!#       $f_n(x) = x^{n+1} I_n(1/x) = n \int_0^1 y^n/(e^{y/x}-1) dy$.
!#
!# Note that, contrary to what is written in Draine & Li 2001 (ApJ 551, 807), the last integral 
!# must be multiplied, not divided, by $n$ to reproduce figure 2 of their paper. This is also 
!# required to have $T_D f_3(T/T_D) = T D_3(T_D/T)$, where 
!#
!#      $D_n(u) = (n/u^n) \int_0^u t^n/(e^t-1) dt$
!#
!# (http://en.wikipedia.org/wiki/Debye_model).
!#
!# For $u = 1/x \gg 1$, $I_n(u)$ tends to a constant $C_n > 0$, so $f_n(x) \approx C_n x^{n+1}$.
!# For $x \gg 1$, $f_n(x) \approx x$.
!#  
          32
 .10000E-04 .10000E-09 .10000E-14
 .10518     .10680E-01 .11184E-02
 .22141     .45506E-01 .99801E-02
 .34987     .10876     .37471E-01
 .49184     .20468     .98470E-01
 .64874     .33720     .21232    
 .82214     .50957     .40300    
 1.0138     .72396     .69870    
 1.2256     .98088     1.1306    
 1.4596     1.2787     1.7308    
 1.7183     1.6134     2.5283    
 2.0042     1.9776     3.5448    
 2.3202     2.3614     4.7885    
 2.6693     2.7520     6.2486    
 3.0552     3.1351     7.8907    
 3.4817     3.4957     9.6555    
 3.9531     3.8205     11.462    
 4.4740     4.0989     13.216    
 5.0497     4.3248     14.825    
 5.6860     4.4976     16.211    
 6.3891     4.6213     17.326    
 7.1663     4.7037     18.158    
 8.0251     4.7543     18.732    
 8.9743     4.7828     19.093    
 10.023     4.7974     19.299    
 11.183     4.8041     19.405    
 12.464     4.8068     19.453    
 13.880     4.8078     19.472    
 15.445     4.8081     19.479    
 17.174     4.8082     19.481    
 19.086     4.8082     19.482    
 21.198     4.8082     19.482    
