More problems in 2016 part 7

d 1000 λ ( d ) + d 330091 ( τ ( d ) μ ( d ) ) = ? \large\sum_{d\mid 1000}\lambda (d) + \large\sum_{d\mid 330091} \left(\tau (d) \cdot \mu (d) \right) = \, ?

Notations

1) μ ( d ) \mu (d) denotes Möbius Function

2) τ ( d ) \tau (d) denotes the divisor function.

3) λ ( d ) \lambda (d) denotes Liouville function


The answer is -1.

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1 solution

Aareyan Manzoor
Jan 1, 2016

all μ , τ , λ \mu,\tau,\lambda are multiplicative functions, implying the summations are as well.

we use multiplicity and the fact that d n λ ( d ) = { 1 if n is a perfect square 0 otherwise \sum_{d|n}\lambda(d)=\begin{cases} 1&&\text{if n is a perfect square}\\0&&\text{otherwise}\end{cases} . the first part becomes zero. and the remaining f ( p k ) = d p k ( μ ( p k ) τ ( p k ) ) = 1 2 + 3 4 + . . . + ( 1 ) k ( k + 1 ) = { k + 2 2 if k is even k 1 2 if k is odd f(p^k)=\sum_{d|p^k}(\mu(p^k)\tau(p^k))=1-2+3-4+...+(-1)^k(k+1)=\begin{cases} \dfrac{k+2}{2} &&\text{if k is even}\\\dfrac{-k-1}{2}&&\text{if k is odd}\end{cases} so f ( 41 ) = 1 , f ( 83 ) = 1 , f ( 97 ) = 1 f(41)=-1,f(83)=-1,f(97)=-1 since f is multiplicative: f ( 330091 ) = f ( 41 ) f ( 83 ) f ( 97 ) = 1 1 1 = 1 f(330091)=f(41)f(83)f(97)=-1*-1*-1=-1 . so the final answe is 0 + ( 1 ) = 1 0+(-1)=\boxed{-1}

A great solution indeed ! (+1)

A Former Brilliant Member - 5 years, 5 months ago

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@Otto Bretscher , @Chew-Seong Cheong , @Jessica Wang , @Dev Sharma Do try this problem though it is quite basic , @Aareyan Manzoor has written a wonderful solution ,,,

A Former Brilliant Member - 5 years, 5 months ago

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Thank you for posting @Chinmay Sangawadekar :)

P.S. For me, it is not "basic"...

Jessica Wang - 5 years, 5 months ago

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