Average $lcm(a,b)$, $ 1\le a \le b \le n$, and asymptotic behavior 
What is the average value for $\mathrm{lcm}(a,b)$, with $ 1\le a \le b \le n$, for a
  given $n$, and what is the asymptotic behavior? The $\mathrm{lcm}$ is the least common multiple.

I have calculated, as $(n, avg)$: 
$$(10, 19.836)$$
$$(100, 1826.859)$$
$$(1000, 182828.976)$$
The values appear to converge quadratically to some constant.
This generalizes to $$\sum_{1 \le j \le k \le n} \mathrm{lcm}(j,k)$$
 A: Here is a back-of-the-envelope calculation.
Start with the average value of $ab$, which is $(n+1)^2/4$.
Consider powers of $2$ only.  Three-quarters of the $(a,b)$ pairs lack $2$ as a common factor.  3/16 have $2^1$ as  a common factor, $3/64$ have $4$ as a common factor and so on.  So, on average, dealing with powers of $2$ reduces the average by a factor $$\frac341+\frac3{16}\frac12+\frac3{64}\frac14+...\\
=\frac34\left[1+\frac18+\frac1{64}+...\right]\\=\frac{3/4}{7/8}=6/7$$
Consider powers of $3$ only, which will be independent from powers of $2$.  The reduction is 
$$\frac891+\frac8{81}\frac13+\frac8{729}\frac19+...=\frac{8/9}{26/27}=\frac{12}{13}$$
For any prime $p$, the factor is $\frac{p^3-p}{p^3-1}$, so my final answer is 
$$\frac{(n+1)^2}4\prod_{p\,\text{prime}}\frac{p^3-p}{p^3-1}=\frac{(n+1)^2}4\frac{\zeta(3)}{\zeta(2)}$$
That is the Riemann $\zeta$ function, and the constant ratio is 
$$\frac{\zeta(3)}{4\zeta(2)}=0.18269074235...$$
A: Theorem 6.3 of Olivier Bordelles, Mean values of generalized gcd-sum and lcm-sum functions, Journal of Integer Sequences, Vol. 10 (2007), Article 07.9.2, says, for any real number $x$ sufficiently large, $$\sum_{n\le x}\sum_{j=1}^n[n,j]={\zeta(3)\over8\zeta(2)}x^4+O\bigl(x^3(\log x)^{2/3}(\log\log x)^{4/3}\bigr)$$
