What is 4n+2k=01k+1+1k+3 ?

1 Answer
Jun 20, 2018

S=2H4n+3+14n+4+14n+532

Explanation:

We seek a way to evaluate the sum

S=mk=0(1k+1+1k+3)

Note:

  • The substitution m=4n+2, is made for simplicity

Definition:

The n-th harmonic number Hn is a number on the form

Hn=nk=11k

Our goal will be to express the original sum in terms of a harmonic number.

It may seems strange to express the original sum, by another sum. But, you may think of the harmonic numbers, similar to the factorial, in the way evaluate them.

Express the sum in terms of a harmonic number:

Using some basic summation identities

S=mk=01k+1+mk=01k+3

S=m+1k=11k+m+3k=31k

S=m+1k=11k+m+3k=11k32

S=2m+1k=11k+1m+2+1m+332

Or in terms of a harmonic number

S=2Hm+1+1m+2+1m+332

For your problem you may substitute back m=4n+2

S=2H4n+3+14n+4+14n+532

Bonus info

A fairly reasonable approximation of a harmonic number is

Hnln(n)γ

The drawing at the upper right hand corner illustrates this quite well