Commutative algebra

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In mathematics, summation by parts transforms the summation of products of sequences into other summations, often simplifying the computation or (especially) estimation of certain types of sums. The summation by parts formula is sometimes called Abel's lemma or Abel transformation.

Statement

Suppose $\{f_{k}\}$ and $\{g_{k}\}$ are two sequences. Then,

$\sum _{k=m}^{n}f_{k}(g_{k+1}-g_{k})=\left[f_{n+1}g_{n+1}-f_{m}g_{m}\right]-\sum _{k=m}^{n}g_{k+1}(f_{k+1}-f_{k}).$

Using the forward difference operator $\Delta$, it can be stated more succinctly as

$\sum _{k=m}^{n}f_{k}\Delta g_{k}=\left[f_{n+1}g_{n+1}-f_{m}g_{m}\right]-\sum _{k=m}^{n}g_{k+1}\Delta f_{k},$

Note that summation by parts is an analogue to the integration by parts formula,

$\int f\,dg=fg-\int g\,df.$

Note also that although applications almost always deal with convergence of sequences, the statement is purely algebraic and will work in any field. It will also work when one sequence is in a vector space, and the other is in the relevant field of scalars.

Newton series

The formula is sometimes given in one of these - slightly different - forms

${\begin{aligned}\sum _{k=0}^{n}f_{k}g_{k}&=f_{0}\sum _{k=0}^{n}g_{k}+\sum _{j=0}^{n-1}(f_{j+1}-f_{j})\sum _{k=j+1}^{n}g_{k}\\&=f_{n}\sum _{k=0}^{n}g_{k}-\sum _{j=0}^{n-1}\left(f_{j+1}-f_{j}\right)\sum _{k=0}^{j}g_{k},\end{aligned}}$

which represent a special cases ($M=1$) of the more general rule

${\begin{aligned}\sum _{k=0}^{n}f_{k}g_{k}&=\sum _{i=0}^{M-1}f_{0}^{(i)}G_{i}^{(i+1)}+\sum _{j=0}^{n-M}f_{j}^{(M)}G_{j+M}^{(M)}=\\&=\sum _{i=0}^{M-1}\left(-1\right)^{i}f_{n-i}^{(i)}{\tilde {G}}_{n-i}^{(i+1)}+\left(-1\right)^{M}\sum _{j=0}^{n-M}f_{j}^{(M)}{\tilde {G}}_{j}^{(M)};\end{aligned}}$

both result from iterated application of the initial formula. The auxiliary quantities are Newton series:

$f_{j}^{(M)}:=\sum _{k=0}^{M}\left(-1\right)^{M-k}{M \choose k}f_{j+k}$

and

$G_{j}^{(M)}:=\sum _{k=j}^{n}{k-j+M-1 \choose M-1}g_{k},$

${\tilde {G}}_{j}^{(M)}:=\sum _{k=0}^{j}{j-k+M-1 \choose M-1}g_{k}.$

A remarkable, particular ($M=n+1$) result is the noteworthy identity

$\sum _{k=0}^{n}f_{k}g_{k}=\sum _{i=0}^{n}f_{0}^{(i)}G_{i}^{(i+1)}=\sum _{i=0}^{n}(-1)^{i}f_{n-i}^{(i)}{\tilde {G}}_{n-i}^{(i+1)}.$

Here, ${n \choose k}$ is the binomial coefficient.

Method

For two given sequences $(a_{n})\,$ and $(b_{n})\,$, with $n\in \mathbb {N}$, one wants to study the sum of the following series:
$S_{N}=\sum _{n=0}^{N}a_{n}b_{n}$

If we define $B_{n}=\sum _{k=0}^{n}b_{k},$  then for every $n>0,\,$  $b_{n}=B_{n}-B_{n-1}\,$  and

$S_{N}=a_{0}b_{0}+\sum _{n=1}^{N}a_{n}(B_{n}-B_{n-1}),$

$S_{N}=a_{0}b_{0}-a_{1}B_{0}+a_{N}B_{N}+\sum _{n=1}^{N-1}B_{n}(a_{n}-a_{n+1}).$

Finally  $S_{N}=a_{N}B_{N}-\sum _{n=0}^{N-1}B_{n}(a_{n+1}-a_{n}).$

This process, called an Abel transformation, can be used to prove several criteria of convergence for $S_{N}\,$ .

Similarity with an integration by parts

The formula for an integration by parts is $\int _{a}^{b}f(x)g'(x)\,dx=\left[f(x)g(x)\right]_{a}^{b}-\int _{a}^{b}f'(x)g(x)\,dx$
Beside the boundary conditions, we notice that the first integral contains two multiplied functions, one which is integrated in the final integral ( $g'\,$ becomes $g\,$ ) and one which is differentiated ( $f\,$ becomes $f'\,$ ).

The process of the Abel transformation is similar, since one of the two initial sequences is summed ( $b_{n}\,$ becomes $B_{n}\,$ ) and the other one is differenced ( $a_{n}\,$ becomes $a_{n+1}-a_{n}\,$ ).

Applications

• Summation by parts is frequently used to prove Abel's theorem.

• If $\sum b_{n}$ is a convergent series, and $a_{n}$ a monotone sequence decreasing to zero, then $S_{N}=\sum _{n=0}^{N}a_{n}b_{n}$ remains a convergent series.

The Cauchy criterion gives $S_{M}-S_{N}=a_{M}B_{M}-a_{N}B_{N}+\sum _{n=N}^{M-1}B_{n}(a_{n+1}-a_{n})$.

As $\sum b_{n}$ is convergent, $B_{N}$ is bounded independently of $N$, say by $B$. As $a_{n}$ go to zero, so go the first two terms. The remaining sum is bounded by

$\sum _{n=N}^{M-1}|B_{n}||a_{n+1}-a_{n}|\leq B\sum _{n=N}^{M-1}|a_{n+1}-a_{n}|=B(a_{N}-a_{M})$

by the monotonicity of $a_{n}$, and also goes to zero as $N\to \infty$.

• Using the same proof as above, one shows that

1. if the partial sums $B_{N}$ remain bounded independently of $N$ ;
2. if $\sum _{n=0}^{\infty }|a_{n+1}-a_{n}|<\infty$ (so that the sum $\sum _{n=N}^{M-1}|a_{n+1}-a_{n}|$ goes to zero as $N$ goes to infinity) ; and
3. if $\lim a_{n}=0$

then $S_{N}=\sum _{n=0}^{N}a_{n}b_{n}$ is a convergent series.

In both cases, the sum of the series verifies: $|S|=\left|\sum _{n=0}^{\infty }a_{n}b_{n}\right|\leq B\sum _{n=0}^{\infty }|a_{n+1}-a_{n}|$

See also

• Convergent series
• Divergent series
• Integration by parts
• Cesàro summation
• Abel's theorem
• Abel sum formula

References

• Template:Planetmathref