Please Excuse My Dear Ant Brilli

If Brilli the Ant starts at point A A and crawls up 1 1 p \displaystyle \frac{1}{1^p} units, then right 1 2 p \displaystyle \frac{1}{2^p} units, then up 1 3 p \displaystyle \frac{1}{3^p} units, then right 1 4 p \displaystyle \frac{1}{4^p} units, and continues to crawl in this pattern indefinitely, he will get closer and closer to point B B .

If p p is an integer and the slope of A B AB is 1 16 \displaystyle \frac{1}{16} of a perfect number , find p p .


The answer is 5.

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3 solutions

Slope of the line segment A B \overline {AB} is 1 1 p + 1 3 p + 1 5 p + . . . 1 2 p + 1 4 p + 1 6 p + . . . = 2 p 1 = 1 16 × 496 \dfrac{\dfrac{1}{1^p}+\dfrac{1}{3^p}+\dfrac{1}{5^p}+...}{\dfrac{1}{2^p}+\dfrac{1}{4^p}+\dfrac{1}{6^p}+...}=2^p-1=\dfrac{1}{16}\times {496} . 496 496 is a perfect number. So 2 p 1 = 31 2^p-1=31 or 2 p = 32 = 2 5 2^p=32=2^5 . Hence p = 5 p=\boxed 5 .

(If p p and 2 p 1 2^p-1 be primes, then 2 p 1 ( 2 p 1 ) 2^{p-1}(2^p-1) is a perfect number. So, for prime p , 2 p 1 p, 2^p-1 is ( 1 2 p 1 ) t h (\dfrac{1}{2^{p-1}})^{th} of a perfect number. Hence 2 p 1 = 16 = 2 4 2^{p-1}=16=2^4 , and from here, p = 5 p=5 , a prime number.)

Hypergeo H.
Apr 10, 2020

Assume an unorthodox notation, where n = 1 n p n' = \frac 1{n^p} , e.g. 2 = 1 2 p 2' = \frac 1{2^p} .
Let EV = n = 0 1 ( 2 n ) p = 2 + 4 + 6 + 8 + \displaystyle \text{EV}=\sum_{n=0}^\infty \frac 1{(2n)^p} = 2'+4'+6'+8'+\cdots
and OD = n = 0 1 ( 2 n 1 ) p = 1 + 3 + 5 + 7 + \displaystyle\text{OD}=\sum_{n=0}^\infty \frac 1{(2n-1)^p} =1' +3'+5'+7'+\cdots .

Note that

EV OD = 2 + 4 + 6 + 8 + 1 + 3 + 5 + 7 + = 2 ( 1 + 2 + 3 + 4 + ) 1 + 3 + 5 + 7 + = 2 [ OD + EV ] OD = 2 ( 1 + EV OD ) ( 1 2 ) EV OD = 2 Slope of A B = OD EV = 1 2 2 = 1 2 1 = 2 p 1 \begin{aligned}\\ \frac {\text{EV}}{\text{OD}} &=\frac {2'+4'+6'+8'+\dots} {1'+3'+5'+7+\cdots} \\\\ &=\frac {2' (1'+2'+3'+4'+\dots) } {1'+3'+5'+7+\cdots} \\\\ &=\frac {2' \big[\text{OD}+\text{EV} \big] } {\text{OD}} \\\\ &=2'\left(1+\frac {\text{EV}}{\text{OD}} \right) \\\\ (1-2')\frac {\text{EV}}{\text{OD} } &= 2'\\\\ \text {Slope of }AB &=\frac {\text{OD}}{\text{EV} }\\\\ &=\frac {1-2'}{2'}\\\\ &=\frac 1{2'}-1\\\\ &=2^p-1 \end{aligned}

It is given that
Slope of A B = 1 16 N ( N = perfect number ) 2 p 1 = 1 16 2 q 1 ( 2 q 1 ) N ( q prime ) \begin{aligned} \text{Slope of} AB &= \frac 1{16} N &&(N = \text{perfect number})\\ \therefore 2^p-1 &=\frac 1{16} \cdot \overbrace{2^{q-1} (2^q-1)}^N &&(q \text{ prime}) \end{aligned}

Therefore, 2 q 1 = 16 2^{q-1}=16 and p = q p=q i.e. p ( = q ) = 5 p(=q)=\color{#D61F06}5

David Vreken
Feb 1, 2020

The ant goes up Δ y = n = 1 1 ( 2 n 1 ) p \Delta y = \displaystyle \sum_{n=1}^{\infty} \frac{1}{(2n - 1)^p} and right Δ x = n = 1 1 ( 2 n ) p \Delta x = \displaystyle \sum_{n=1}^{\infty} \frac{1}{(2n)^p} .

Let k = n = 1 1 n p k = \displaystyle \sum_{n=1}^{\infty} \frac{1}{n^p} . Then Δ x = n = 1 1 ( 2 n ) p = 1 2 p n = 1 1 n p = 1 2 p k \Delta x = \displaystyle \sum_{n=1}^{\infty} \frac{1}{(2n)^p} = \frac{1}{2^p}\displaystyle \sum_{n=1}^{\infty} \frac{1}{n^p} = \frac{1}{2^p}k , and Δ y = k Δ x = k 1 2 p k = 2 p 1 2 p k \Delta y = k - \Delta x = k - \frac{1}{2^p}k = \displaystyle \frac{2^p - 1}{2^p}k .

The slope of A B AB is therefore m = Δ y Δ x = 2 p 1 2 p k 1 2 p k = 2 p 1 m = \frac{\Delta y}{\Delta x} = \displaystyle \frac{\frac{2^p - 1}{2^p}k}{\frac{1}{2^p}k} = 2^p - 1 .

Since the slope m m is equal to 1 16 \frac{1}{16} of a perfect number P P , then P = 16 m = 16 ( 2 p 1 ) P = 16m = 16(2^p - 1) .

Since all even perfect numbers are in the form of 2 p 1 ( 2 p 1 ) 2^{p - 1}(2^p - 1) and 16 16 is its only even factor, 16 = 2 p 1 16 = 2^{p - 1} , and this solves to p = 5 p = \boxed{5} .

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