If Brilli the Ant starts at point
A
and crawls up
1
p
1
units, then right
2
p
1
units, then up
3
p
1
units, then right
4
p
1
units, and continues to crawl in this pattern indefinitely, he will get closer and closer to point
B
.
If p is an integer and the slope of A B is 1 6 1 of a perfect number , find p .
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Assume an unorthodox notation, where
n
′
=
n
p
1
, e.g.
2
′
=
2
p
1
.
Let
EV
=
n
=
0
∑
∞
(
2
n
)
p
1
=
2
′
+
4
′
+
6
′
+
8
′
+
⋯
and
OD
=
n
=
0
∑
∞
(
2
n
−
1
)
p
1
=
1
′
+
3
′
+
5
′
+
7
′
+
⋯
.
Note that
OD EV ( 1 − 2 ′ ) OD EV Slope of A B = 1 ′ + 3 ′ + 5 ′ + 7 + ⋯ 2 ′ + 4 ′ + 6 ′ + 8 ′ + … = 1 ′ + 3 ′ + 5 ′ + 7 + ⋯ 2 ′ ( 1 ′ + 2 ′ + 3 ′ + 4 ′ + … ) = OD 2 ′ [ OD + EV ] = 2 ′ ( 1 + OD EV ) = 2 ′ = EV OD = 2 ′ 1 − 2 ′ = 2 ′ 1 − 1 = 2 p − 1
It is given that
Slope of
A
B
∴
2
p
−
1
=
1
6
1
N
=
1
6
1
⋅
2
q
−
1
(
2
q
−
1
)
N
(
N
=
perfect number
)
(
q
prime
)
Therefore, 2 q − 1 = 1 6 and p = q i.e. p ( = q ) = 5
The ant goes up Δ y = n = 1 ∑ ∞ ( 2 n − 1 ) p 1 and right Δ x = n = 1 ∑ ∞ ( 2 n ) p 1 .
Let k = n = 1 ∑ ∞ n p 1 . Then Δ x = n = 1 ∑ ∞ ( 2 n ) p 1 = 2 p 1 n = 1 ∑ ∞ n p 1 = 2 p 1 k , and Δ y = k − Δ x = k − 2 p 1 k = 2 p 2 p − 1 k .
The slope of A B is therefore m = Δ x Δ y = 2 p 1 k 2 p 2 p − 1 k = 2 p − 1 .
Since the slope m is equal to 1 6 1 of a perfect number P , then P = 1 6 m = 1 6 ( 2 p − 1 ) .
Since all even perfect numbers are in the form of 2 p − 1 ( 2 p − 1 ) and 1 6 is its only even factor, 1 6 = 2 p − 1 , and this solves to p = 5 .
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Slope of the line segment A B is 2 p 1 + 4 p 1 + 6 p 1 + . . . 1 p 1 + 3 p 1 + 5 p 1 + . . . = 2 p − 1 = 1 6 1 × 4 9 6 . 4 9 6 is a perfect number. So 2 p − 1 = 3 1 or 2 p = 3 2 = 2 5 . Hence p = 5 .
(If p and 2 p − 1 be primes, then 2 p − 1 ( 2 p − 1 ) is a perfect number. So, for prime p , 2 p − 1 is ( 2 p − 1 1 ) t h of a perfect number. Hence 2 p − 1 = 1 6 = 2 4 , and from here, p = 5 , a prime number.)