Who's up to the challenge?17

exp ( n 896 Λ ( n ) ) = ? \exp \left ( {\displaystyle \sum _{ n|896 }{ \Lambda (n) } } \right) = \, ?

Notations :

  • Λ ( x ) \Lambda(x) denotes the Mangoldt function .

  • exp ( x ) \exp(x) denotes the exponential function, exp ( x ) = e x \exp(x) = e^x .


this is a part of Who's up to the challenge?


The answer is 896.

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1 solution

Soumava Pal
Mar 22, 2016

By the definition of the Mangoldt Function we know that λ ( n ) = l o g ( p ) \lambda(n)=log(p) if n = p k n=p^k , where p p is a prime and k 1 k \ge 1 is a natural number and 0 0 otherwise.

So we have d n λ ( d ) = l o g ( n ) \sum_{d|n}^{} \lambda(d)=log(n) which follows from the Fundamental Theorem of Arithmetic .

Let n = p 1 α 1 . . . p t α t n={p_1}^{\alpha_1}*...*{p_t}^{\alpha_t} be the canonical decomposition of n n so that the p i p_i 's are the primes and α i \alpha_i 's are the powers of the primes.

Now s u m d n λ ( d ) sum_{d|n}^{} \lambda(d) will be same as the sum of λ ( p i β i ) \lambda({p_i}^{\beta_i}) for all 0 β i α i 0 \le \beta_i \le \alpha_i , so it will be equal to

α 1 l o g ( p 1 ) + α 2 l o g ( p 2 ) + . . . + α t l o g ( p t ) {\alpha_1}*{log({p_1})}+{\alpha_2}*{log({p_2})}+...+{\alpha_t}*{log({p_t})}

= l o g ( p 1 α 1 . . . p t α t ) =log({p_1}^{\alpha_1}*...*{p_t}^{\alpha_t})

= l o g ( n ) =log(n)

So the sum in the power of e e in the above expression is same as l o g ( n ) log(n) .

Hence the answer is n = 896 n=896 .

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