Is there a function that gives the same result for a number and its reciprocal?

Question

Is there a (non-piecewise, non-trivial) function where $f(x) = f(\frac{1}{x})$?

Why?

It would be nice to compare ratios without worrying about the ordering of numerator and denominator. For example, I might want to know whether the "magnitude" of the ratio (maybe the "absolute ratio") of the widths of two objects is greater than $2$, but not care which is larger.

It occurred to me that there's a common solution for this problem when comparing the difference of two numbers: the square of a number is the same as the square of its opposite - $(a-b)^2=(b-a)^2$. This is really useful with Euclidean distances, because you don't have to worry about the order of subtraction or use absolute values. Can we get the same elegance for ratios?

Difference: $g(a-b)=g(b-a) \rightarrow g(x)=x^2$

Ratio: $f(\frac{a}{b})=f(\frac{b}{a}) \rightarrow f(x)=\ ?$

For positive $x$, we have $(\ln(x))^2=(\ln(1/x))^2$. So $f(t)=(\ln(t))^2$ works. But it may not be what you are after. — André Nicolas, Nov 25 '13 at 04:35
@StephenMontgomery-Smith that's not a very useful solution, but kudos for it anyway :) — Sparr, Nov 25 '13 at 04:43
@Sparr I'm calling that one trivial: it's (nearly) $f(x)=1$ ...which also works — Nolan Amy, Nov 25 '13 at 04:58
Let $f(x)=whatever$ for $|x|>=1$ or $x=0$, and define $f$ on the rest of its domain as $f(x)=f(1/x)$. — user2357112, Nov 25 '13 at 11:16
@CarstenSchultz I was worried about that. Not exactly fluent in LaTeX or precise math notation... feel free to edit or suggest a change. — Nolan Amy, Nov 25 '13 at 20:19

adam W · Accepted Answer · 2013-11-25T06:06:35.537

12

$$ \frac{x}{x^2+1} $$ the inverse of Will Jagy's $x + \frac{1}{x}$.

I like $$x - \frac{1}{x} = \frac{x^2-1}{x}$$ because it contains sign information (input magnitude greater or less than one) that you may choose to ignore, and gives a nice zero for $x = \frac{1}{x} = \pm 1$. If you choose to take the absolute value of it (ignoring the sign) it gives you the desired $f(x) = f(\frac{1}{x})$

edited Nov 25 '13 at 06:06

answered Nov 25 '13 at 04:55

adam W

5,565

7

For that matter, any $f(x) + f(\frac{1}{x})$, or $f(x)\cdot f(\frac{1}{x})$ – adam W Nov 25 '13 at 04:59
Oooh that's nice – Nolan Amy Nov 25 '13 at 06:36
4

...or simply any $f(x+\frac1x)$ (or, if you want adamW's sign dependency, $f(x-\frac1x)$) – Tobias Kienzler Nov 25 '13 at 10:38

score 11 · Answer 2 · answered Nov 25 '13 at 06:12

11

For lack of a better idea, $f(x)=|\ln x|$.

answered Nov 25 '13 at 06:12

Lucian

48,334
2
83
154

score 8 · Answer 3 · edited Nov 25 '13 at 08:59

8

$$ \frac{1}{1 + x + \frac{1}{x}} = \frac{x}{x^2 + x + 1} $$ extends to the real analytic function written on the right. Notice that $x^2 + x + 1 = (x + \frac{1}{2})^2 + \frac{3}{4} \geq \frac{3}{4}. $

edited Nov 25 '13 at 08:59

Dietrich Epp

123

answered Nov 25 '13 at 04:42

Will Jagy

139,541

score 2 · Answer 4 · answered Nov 25 '13 at 08:23

2

For lack of anything worse than this, $f(x)=(x - \frac{1}{x})^2$.

answered Nov 25 '13 at 08:23

Jardine

121

score 2 · Answer 5 · answered Nov 25 '13 at 10:35

2

Let $f(x)$ be arbitrary for $-1\leqslant x\leqslant 1,$ and define $f(x):=f(1/x)$ otherwise.

answered Nov 25 '13 at 10:35

John Bentin

18,454

The question specifies "non-piecewise" – Sparr Nov 25 '13 at 16:17
@Sparr: This definition includes the "non-piecewise" function specified in Will Jagy's answer as well as the first function in adam W's answer. (The other functions require a second statement to define them at $0.$) In fact, it includes all functions $f$ that satisfy $f(x)=f(1/x)$ for $x\neq 0.$ – John Bentin Nov 25 '13 at 18:18
1

one of the pieces of your function being non-piecewise does not make your function non-piecewise. pretty much the opposite, actually – Sparr Nov 25 '13 at 19:30
@Sparr: Agreed. – John Bentin Nov 25 '13 at 21:26

Dan · Answer 6 · 2021-12-06T20:30:18.303

1

It works for any function $f(x) = g(x, \frac{1}{x})$ where $g$ is symmetric (i.e., $g(x, y) = g(y, x)$).

A simple example is to have $g$ be the addition operator, so $f(x) = x + \frac{1}{x}$.

edited Dec 06 '21 at 20:30

answered Nov 25 '13 at 17:00

Dan

14,978

OP wants actual functions – Cruncher Nov 25 '13 at 19:02

Is there a function that gives the same result for a number and its reciprocal?

Why?

6 Answers6