I've written about Euler's Totient Function in an eponymously titled post on March 12th 2019. Today however, my diurnal age number count of 26248 alerted me to the fact that this number was a member of OEIS A333020:
A333020 | Starts of runs of 3 consecutive even numbers that are all totient numbers (A002202). |
A totient number is a value of Euler's totient function: that is, an \(m\) for which there is at least one \(n\) for which \( \phi(n) = m\). The valency or multiplicity of a totient number \(m\) is the number of solutions to this equation. A non-totient is a natural number which is not a totient number. Source.
Apart from 1, every totient number is even. This follows from the fact that every natural number is either prime or a product of primes and the totient of a prime number is one less than the number itself. For example, \( \phi(7)=6\) because 1, 2, 3, 4, 5 and 6 are coprime to it. All primes except 2 are odd and so the totient of all odd primes is even. Let's look at the totient numbers between 1 and 100:
1, 2, 4, 6, 8, 10, 12, 16, 18, 20, 22, 24, 28, 30, 32, 36, 40, 42, 44, 46, 48, 52, 54, 56, 58, 60, 64, 66, 70, 72, 78, 80, 82, 84, 88, 92, 96, 100
A001274 | Numbers \(k\) such that \( \phi(k) = \phi(k+1) \). |
1, 3, 15, 104, 164, 194, 255, 495, 584, 975, 2204, 2625, 2834, 3255, 3705, 5186, 5187, 10604, 11715, 13365, 18315, 22935, 25545, 32864, 38804, 39524, 46215, 48704, 49215, 49335, 56864, 57584, 57645, 64004, 65535, 73124, 105524, 107864, 123824, 131144, 164175, 184635, ...
Apparently, in 1999, the following was proved that:
for every integer \(k \geq 2 \) there is a totient number \(m\) of multiplicity \(k\): that is, for which the equation \( \phi(n) = m\) has exactly \(k\) solutions. However, no number \(m\) is known with multiplicity \(k = 1\). Source.
It's interesting to look at the distribution of these \(k\)s. For values of \(m\) up to one million, we find that \(m\)=241920 has a \(k\) value of 937. Figure 1 shows the distribution:
Figure 1: permalink |
As can be seen, \(k\)=937 is quite an outlier. The next highest \(k\) value is 750. This means that up to the one million mark, there are no \(k\) values between 751 to 936 inclusive.
There are many sequences in the OEIS that relate to totient numbers. Here is one example:
A050495 | Numbers that are the first term of at least one arithmetic progression with 4 or more terms all having the same value of Euler's totient function \( \phi(x) \). |
The example is given of \( \phi(x) \)72 = \( \phi(x) \)(78) = \( \phi(x) \)(84) = \( \phi(x) \)(90) = 24, so 72 is a member of the sequence.
Figure 2 shows the totient numbers between 1 and 72, along with the numbers that produce these totient values:
Figure 2: source |
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