Two beautiful mathematical Equations

Gamma

This shows that the sum of the reciprocals of the natural numbers, e.g. 1/1 + 1/2 + 1/3 + 1/4 + 1/5... grows just like the natural logarithm.

γ = n lim p p n 1 p - ln n 0.577215

For details look here.

Meissel Mertens

This shows that the sum of the reciprocals of the prime numbers, e.g. 1/2 + 1/3 + 1/5 + 1/7 + 1/11 ... grows just like the natural logarithm iterated twice.

M = n lim p p n 1 p - ln ln n 0.261497

For details look here.

Convergence

Note that the convergence is quite slow, as the following Mathematica output shows:

            In[1]:= n=1000000;
                    N[Total[1/Select[Range[n],True &]] - Log[    n ]]
                    N[Total[1/Select[Range[n],PrimeQ]] - Log[Log[n]]]
	    Out[2]= 0.577216
	    Out[3]= 0.261536
	  

A final thought

Which subset {a, b, c, d, e, ...} of the natural numbers, when summing up their reciprocals, 1/a + 1/b + 1/c + 1/d + 1/e ... grows just like log(log(log(n)))? Prime twins is not big enough as their sum converges (Cf. Brun's Theorem).