Цитата:
Three no-complete answers to the same question ?
...........................................?.........
My solution depends on "Bernstein polynomials".
Denote

and define
![$ f:[0,1]\to {\mathbb R} \; ,\; f(x)=\frac{1}{x+a} \; .$ $ f:[0,1]\to {\mathbb R} \; ,\; f(x)=\frac{1}{x+a} \; .$](https://dxdy-04.korotkov.co.uk/f/b/6/6/b660b4d1705644060df5ce0375f65b0e82.png)
Let us remind that in fact

is the
Bernstein basis of the vectorial space

of all (real) polynomials of degree

.
Remark: The polynomial
![\[
\left(B_ng\right)(x):= \sum\limits_{k=0}^{n}b_{n,k}(x)g\left(\frac{k}{n}\right) \] \[
\left(B_ng\right)(x):= \sum\limits_{k=0}^{n}b_{n,k}(x)g\left(\frac{k}{n}\right) \]](https://dxdy-02.korotkov.co.uk/f/d/2/2/d22f07769c057d2cee295b0f3e0c73a882.png)
is known as
Bernstein polynomial corresponding to a function
![$g:[0,1]\to {\mathbb R}. $ $g:[0,1]\to {\mathbb R}. $](https://dxdy-01.korotkov.co.uk/f/8/9/7/897d55a7786256e8816d249858d55c8e82.png)
See (S.N.Bernstein, 1912, Harkov) Recognize that

where

Further we have
where
![$ D_{k,n}[f]:=\alpha_k f\left(\frac{k}{n}\right)+\beta_kf\left(\frac{k+1}{n}\right) -
f\left(\frac{k+1}{n+1}\right) $ $ D_{k,n}[f]:=\alpha_k f\left(\frac{k}{n}\right)+\beta_kf\left(\frac{k+1}{n}\right) -
f\left(\frac{k+1}{n+1}\right) $](https://dxdy-01.korotkov.co.uk/f/0/f/3/0f31e8e375cbe2c5c01a63af171df13582.png)
with
Observe that

Moreover
Because for

the function
![$f:[0,1]\to {\mathbb R}$ $f:[0,1]\to {\mathbb R}$](https://dxdy-02.korotkov.co.uk/f/5/c/6/5c6be3bef13792b40461576189a6f5b482.png)
is (strictly)- convex , we have
![$ D_{k,n}[f]>0 $ $ D_{k,n}[f]>0 $](https://dxdy-03.korotkov.co.uk/f/2/d/a/2da49ebc7c6ef04ace8d219d59413d2282.png)
. In conclusion
show us that
Now , if

then according to Jensen inequality we have
![\[
F\left(\sum\limits_{k=0}^{n}w_k\frac{k}{n}\right)\le
\sum\limits_{k=0}^{n}w_kF\left(\frac{k}{n}\right)
\] \[
F\left(\sum\limits_{k=0}^{n}w_k\frac{k}{n}\right)\le
\sum\limits_{k=0}^{n}w_kF\left(\frac{k}{n}\right)
\]](https://dxdy-01.korotkov.co.uk/f/4/f/6/4f6491ed4cd906750dd65ac07e55bf0082.png)
for any convex function
![$F:[0,1]\to {\mathbb R} $ $F:[0,1]\to {\mathbb R} $](https://dxdy-01.korotkov.co.uk/f/c/1/c/c1cbd7e626b6257b3d529c0f25fa137c82.png)
(convex on its domain).
Select

and
![$ w_k=b_{n,k}(x)\; ,\; x\in [0,1].$ $ w_k=b_{n,k}(x)\; ,\; x\in [0,1].$](https://dxdy-04.korotkov.co.uk/f/f/a/d/fad8b2463b347dc1a76710fd96a9bd4982.png)
One finds
Note that in Russia there is a very good school regarding the topic
" Bernstein polynomials": see
S.N.Bernstein (Commun.Soc.Math.Kharkow 13(1912) ),
E.Voronovskaja-1932 ,
L.V.Kantorovich -1930,1931,1934,
A.O.Gelfond -1950,
V.A. Baskakov -1960-1975,
Dzjadijh
P.P. Korovkin e.g. see : " Linear operators and approximation". Fizmatgiz, Moskow
1959, (also the translations: Delhi 1960 and Gordon and Breach, New York,
1960), MR 27 \# 561.
R.G. Mamedov -1959/1967,
N.A. Sapogov-1962/1964 ,
Yu.A. Saskin 1960-1970 ,
S.A. Teljakovskii -1965 ,
Timan ,
A.H. Tureckii -1961. ......