Infinitely Many Rhondas
A positive integer N is a "Rhonda Number" if for some positive integer
B the product of the base-B digits of N equals B times the sum of the
prime factors of N. The smallest example is 560, which is a Rhonda
Number to the base 12.
PROPOSITION: There are infinitely many Rhonda Numbers.
PROOF: Let sopf(n) denote the sum of the prime factors of n. Then
for any integer m>5 the number N = km(m+1)(2m+1)^2 is a Rhonda Number
to the base B = 2km(m+1), where k is any integer such that
sopf(k) = m(m+1) - sopf(m) - sopf(m+1) - 2 sopf(2m+1)
We are assured the existence of at least one k satisfying this
equation, because sopf(n) <= n, which implies the negative terms on
the right side combined can be no larger than 6m+3, which is smaller
than m(m+1) for m>5. Therefore, the above expression for sopf(k)
is a positive integer u. If u is even then u=2s and k=2^s satisfies
the equation. If u is odd, then u=2s+3 and k=3(2^s) satisfies the
equation.
The digits of N in the base B are d0 = km(m+1) and d1 = 2m(m+1), so
the product of these digits is 2k(m^2)(m+1)^2. Also, since the sopf
function is additive, we have
sopf(N) = sopf(k) + sopf(m) + sopf(m+1) + 2 sopf(2m+1)
= m(m+1)
so B times sopf(N) equals 2k(m^2)(m+1)^2, which equals the product of
the base-B digits as required.
In general, for each value of m there correspond several distinct
Rhonda Numbers, one for each prime partition of sopf(k). The smallest
example of a Rhonda Number given by this construction is 28392
relative to the base 336 (corresponding to m=6 and k=4).
By the way, the number 140800 has an interesting property that I've
never heard anyone mention. The digits of this number when written in
each of several bases are shown below:
Base d2 d1 d0
198 3 117 22
832 169 192
1200 117 400
1540 91 660
1728 81 832
2024 69 1144
2360 59 1560
2720 51 2080
In each case the product of the digits divided by the base equals
39, which also happens to be the sum of the prime factors of 140800.
Does anyone know of a number with this property relative to more
than 8 distinct bases?
Return to MathPages Main Menu
Сайт управляется системой
uCoz