Given that
g(x)=logf(x)
where f(x) is a twice differentiable positive function on (0,∞) satisfying the functional relation:
f(x+1)=xf(x)
Taking the natural logarithm on both sides of the relation, we get:
logf(x+1)=log(x⋅f(x))
⟹logf(x+1)=logx+logf(x)
Substituting g(x)=logf(x) into the equation gives:
g(x+1)=logx+g(x)
Differentiating both sides with respect to x:
g′(x+1)=x1+g′(x)
Differentiating again with respect to x:
g′′(x+1)=−x21+g′′(x)
⟹g′′(x+1)−g′′(x)=−x21
To evaluate g′′(N+21)−g′′(21), we substitute x=k+21 for k=0,1,2,…,N−1:
g′′(k+23)−g′′(k+21)=−(k+21)21=−(2k+1)24
Summing both sides from k=0 to N−1:
∑k=0N−1[g′′(k+23)−g′′(k+21)]=∑k=0N−1[−(2k+1)24]
The left-hand side forms a telescoping series:
[g′′(23)−g′′(21)]+[g′′(25)−g′′(23)]+⋯+[g′′(N+21)−g′′(N−21)]
=g′′(N+21)−g′′(21)
Evaluating the right-hand side sum:
−4∑k=0N−1(2k+1)21=−4{1+91+251+⋯+(2N−1)21}
Thus,
g′′(N+21)−g′′(21)=−4{1+91+251+⋯+(2N−1)21}
Hence, the correct option is (A).