

A343073


a(n) is the number of integers 0 < b < n such that b^^x == 1 (mod n) has a solution.


1



1, 2, 1, 3, 1, 2, 1, 2, 1, 3, 1, 6, 2, 5, 1, 9, 1, 5, 1, 3, 3, 2, 1, 3, 3, 2, 2, 5, 1, 3, 1, 5, 1, 8, 1, 9, 2, 5, 1, 8, 1, 6, 3, 5, 1, 2, 1, 4, 1, 17, 2, 5, 1, 5, 2, 3, 3, 3, 1, 7, 3, 3, 1, 15, 2, 5, 1, 5, 2, 4, 1, 16, 4, 5, 3, 10, 1, 5
(list;
graph;
refs;
listen;
history;
text;
internal format)



OFFSET

2,2


COMMENTS

If the same definition were used, but with b^x instead of b^^x, then a(n) would be A000010(n), the Euler Totient Function.
A019434 plays a special role for this sequence. a(A019434(n)) = (A019434(n)+1)/2, since all even numbers b satisfy the condition, and b=1 is the only odd number that satisfies it. This can be easily proved with the FermatEuler Theorem.
a(n) <= A000010(n), since gcd(b,n)=1 is a necessary condition. There is equality when n = 2 and n = 3. It is a conjecture that there are no more equality cases.
The sequence A239063 seems to give exactly the numbers n where a(n) = 1. This means that if b^^2 == 1 (mod n) has no solutions with 1 < b < n, then neither will b^^x == 1 (mod n). However, this needs to be proved.


LINKS

Bernat Pagès Vives, Table of n, a(n) for n = 2..500
Wikipedia, Tetration
Wikipedia, Carmichael Function


FORMULA

If n is a Fermat prime, a(n) = (n+1)/2.
If n is a power of 2, a(n) = 1.


EXAMPLE

For n = 5,
Setting b = 1, x = 1 gives 1^^1 == 1 (mod 5).
Setting b = 2, x = 3 gives 2^^3 == 2^8 == 1 (mod 5).
Setting b = 3 has no solutions, since 3^^x == 2 (mod 5) for all x > 1.
Setting b = 4, x = 2 gives 4^^2 == 1 (mod 5).
Thus, there are 3 possible values of b, and that is the value of a(5).


MATHEMATICA

Tetration[a_, b_, mod_]:=
Which[
Mod[a, mod]==0, 0,
b == 1, Mod[a, mod],
b==2, PowerMod[a, a, mod],
b==3&&a==2, Mod[16, mod],
True, PowerMod[a, Mod[(Tetration[a, b1, EulerPhi[mod]]Floor[Log[2, mod]]), EulerPhi[mod]]+Floor[Log[2, mod]], mod]]
TetraInv[n_, mod_, it_]:=
Which[
GCD[n, mod]!=1 , 0,
it==LambdaRoot[mod]+1, 0,
Tetration[n, it, mod]==1, it,
True, TetraInv[n, mod, it+1]
]
LambdaRoot[n_]:=Module[{counter, it},
counter = 0;
it = n;
While[it!=1,
it = CarmichaelLambda[it];
counter++;
];
counter
]
a[n_] := Module[{counter , t},
counter = 0;
For[j=1, j<=n, j++,
t =TetraInv[j, n, 1];
If[t!=0, counter++]
];
counter
]


CROSSREFS

Cf. A000010, A019434, A239063, A317905.
Sequence in context: A089242 A185894 A214180 * A184166 A029423 A184169
Adjacent sequences: A343070 A343071 A343072 * A343074 A343075 A343076


KEYWORD

nonn


AUTHOR

Bernat Pagès Vives, Apr 04 2021


STATUS

approved



