# Looking for suggestions on how to proceed with showing that:

for $x \ge 2863:$

$$\ln\left(\left\lfloor\frac{x}{6}\right\rfloor!\right) < \sum_{k=5}^{\infty}-\mu(k)\ln\left(\left\lfloor\frac{x}{k}\right\rfloor!\right)$$

I've written a java application which checked from 2,863 to 24,600 which is how I came up with $2,863$.

I'm looking for tips on how to proceed to prove or disprove this comparison.

Since:

$$\ln\left(\left\lfloor\frac{x}{5}\right\rfloor!\right) > \ln\left(\left\lfloor\frac{x}{6}\right\rfloor!\right)$$

The problems comes down to showing that for $x \ge 2863$:

$$\sum_{k=6}^{\infty}\mu(k)\ln\left(\left\lfloor\frac{x}{k}\right\rfloor!\right) \le 0$$

or showing that:

$$\sum_{k=6}^{\infty}\mu(k)\ln\left(\left\lfloor\frac{x}{k}\right\rfloor!\right) \le \ln\left(\left\lfloor\frac{x}{5}\right\rfloor!\right) - \ln\left(\left\lfloor\frac{x}{6}\right\rfloor!\right)$$

Does anyone have any suggestions?

Thanks very much!

-Larry

• You are essentially trying to prove bounds on chebyshevs prime counting function, I don't think this representation for $\psi(x)$ will prove useful. – Ethan May 11 '13 at 21:19
• Thanks, Ethan! I was trying to understand if this representation was useful or not. – Larry Freeman May 11 '13 at 21:59

Did you mean $$\sum_{k=6}^\infty \mu(k) \ln\left(\left\lfloor \frac{x}{k} \right\rfloor !\right) \geq 0$$

If not, I can't reproduce your verification for this inequality using the following Matlab code:

function s = larry( x )

% Compute \sum_{k=6}^\infty \mu(k) \ln\left(\lfloor \frac{x}{k} \rfloor !\right)
s = 0;
for k = 6:ceil(x/2)
s = s + moebiusmu(k)*logfactorial( floor(x/k) );
end

% Compute \mu(k)
function mu = moebiusmu(n)

if n == 1, mu = 1; return; end

p = factor(n);
r = histc(p, unique(p));

if all(r < 2)
mu = (-1)^length(unique(p));
else
mu = 0;
end
end

% Compute \ln\left(\lfloor \frac{x}{k} \rfloor !\right)
function p = logfactorial(n)
if n < 2
p = 0;
else
p = sum(log( 2:n ));
end
end

end

• Thanks. I didn't know how to check the expression. I'll take a look at purchasing a Student License for MatLab for the future. :-) – Larry Freeman May 11 '13 at 22:27
• :) I'm not sure I could come up with a Java version given my poor skills in this language.. C++ maybe if you're interested? – Sheljohn May 11 '13 at 22:29
• I think that your Matlab code is very clear. :-) I just meant that I should probably pick up a MatLab license and start using that instead of Java. :-) – Larry Freeman May 11 '13 at 22:48