JEE Challenger
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Evaluate Difference of Second Derivatives of Logarithmic Gamma Related Function

Let g(x)=log⁡f(x)g(x) = \log f(x) where f(x)f(x) is a twice differentiable positive function on (0,∞)(0, \infty) such that f(x+1)=xf(x)f(x + 1) = x f(x). Then, for N=1,2,3,…N = 1, 2, 3, \dots,
g′′(N+12)−g′′(12)=g''\left(N + \frac{1}{2}\right) - g''\left(\frac{1}{2}\right) =

Options

A

−4{1+19+125+⋯+1(2N−1)2}-4\left\{1 + \frac{1}{9} + \frac{1}{25} + \dots + \frac{1}{(2N - 1)^2}\right\}

Correct
B

4{1+19+125+⋯+1(2N−1)2}4\left\{1 + \frac{1}{9} + \frac{1}{25} + \dots + \frac{1}{(2N - 1)^2}\right\}

C

−4{1+19+125+⋯+1(2N+1)2}-4\left\{1 + \frac{1}{9} + \frac{1}{25} + \dots + \frac{1}{(2N + 1)^2}\right\}

D

4{1+19+125+⋯+1(2N+1)2}4\left\{1 + \frac{1}{9} + \frac{1}{25} + \dots + \frac{1}{(2N + 1)^2}\right\}

Step-by-Step Solution

Given that
g(x)=log⁡f(x)g(x) = \log f(x)
where f(x)f(x) is a twice differentiable positive function on (0,∞)(0, \infty) satisfying the functional relation:
f(x+1)=xf(x)f(x + 1) = x f(x)

Taking the natural logarithm on both sides of the relation, we get:
log⁡f(x+1)=log⁡(x⋅f(x))\log f(x + 1) = \log(x \cdot f(x))
  ⟹  log⁡f(x+1)=log⁡x+log⁡f(x)\implies \log f(x + 1) = \log x + \log f(x)

Substituting g(x)=log⁡f(x)g(x) = \log f(x) into the equation gives:
g(x+1)=log⁡x+g(x)g(x + 1) = \log x + g(x)

Differentiating both sides with respect to xx:
g′(x+1)=1x+g′(x)g'(x + 1) = \frac{1}{x} + g'(x)

Differentiating again with respect to xx:
g′′(x+1)=−1x2+g′′(x)g''(x + 1) = -\frac{1}{x^2} + g''(x)
  ⟹  g′′(x+1)−g′′(x)=−1x2\implies g''(x + 1) - g''(x) = -\frac{1}{x^2}

To evaluate g′′(N+12)−g′′(12)g''\left(N + \frac{1}{2}\right) - g''\left(\frac{1}{2}\right), we substitute x=k+12x = k + \frac{1}{2} for k=0,1,2,…,N−1k = 0, 1, 2, \dots, N-1:

g′′(k+32)−g′′(k+12)=−1(k+12)2=−4(2k+1)2g''\left(k + \frac{3}{2}\right) - g''\left(k + \frac{1}{2}\right) = -\frac{1}{\left(k + \frac{1}{2}\right)^2} = -\frac{4}{(2k + 1)^2}

Summing both sides from k=0k = 0 to N−1N - 1:
∑k=0N−1[g′′(k+32)−g′′(k+12)]=∑k=0N−1[−4(2k+1)2]\sum_{k=0}^{N-1} \left[ g''\left(k + \frac{3}{2}\right) - g''\left(k + \frac{1}{2}\right) \right] = \sum_{k=0}^{N-1} \left[ -\frac{4}{(2k + 1)^2} \right]

The left-hand side forms a telescoping series:
[g′′(32)−g′′(12)]+[g′′(52)−g′′(32)]+⋯+[g′′(N+12)−g′′(N−12)]\left[g''\left(\frac{3}{2}\right) - g''\left(\frac{1}{2}\right)\right] + \left[g''\left(\frac{5}{2}\right) - g''\left(\frac{3}{2}\right)\right] + \dots + \left[g''\left(N + \frac{1}{2}\right) - g''\left(N - \frac{1}{2}\right)\right]
=g′′(N+12)−g′′(12)= g''\left(N + \frac{1}{2}\right) - g''\left(\frac{1}{2}\right)

Evaluating the right-hand side sum:
−4∑k=0N−11(2k+1)2=−4{1+19+125+⋯+1(2N−1)2}-4 \sum_{k=0}^{N-1} \frac{1}{(2k + 1)^2} = -4\left\{1 + \frac{1}{9} + \frac{1}{25} + \dots + \frac{1}{(2N - 1)^2}\right\}

Thus,
g′′(N+12)−g′′(12)=−4{1+19+125+⋯+1(2N−1)2}g''\left(N + \frac{1}{2}\right) - g''\left(\frac{1}{2}\right) = -4\left\{1 + \frac{1}{9} + \frac{1}{25} + \dots + \frac{1}{(2N - 1)^2}\right\}

Hence, the correct option is (A).