Concrete mathematics : a foundation for computer science

The summation
symbol looks like
a distorted pacman.
the delimited form would require us to write
2.1 NOTATION 23
where pk denotes the kth prime and n(N) is the number of primes < N.
(Incidentally, this sum gives the approximate average number of distinct prime
factors of a random integer near N, since about 1 /p of those integers are
divisible by p. Its value for large N is approximately lnln N + 0.261972128;
In x stands for the natural logarithm of x, and In In x stands for ln( In x) .)
The biggest advantage of general Sigma-notation is that we can manipulate
it more easily than the delimited form. For example, suppose we want
to change the index variable k to k + 1. With the general form, we have
ak = ak+l
l