burbery1
(Peter Burbery)
April 11, 2022, 2:44pm
1
I am looking for a way to find an integer that satisfies the statement 5 is divisible by 2^n+1.
The answer is 2 and I have tried Solve, Reduce, and FindInstance and not solved the equation.
Solve[Divisible[5,2^n+1]==True,n]
Reduce[Divisible[5,2^n+1]==True,n]
FindInstance[Divisible[5,2^n+1]==True,n]
Solve[Mod[5,2^n+1]==0,n]
Reduce[Mod[5,2^n+1]==0,n]
FindInstance[Mod[5,2^n+1]==0,n]
Is there a way to solve for the answer of 2?
This way you also get a negative answer:
In[83]:= Solve[Mod[5, 2^n + 1] == 0, n, Integers]
Out[83]= {{n -> -2}, {n -> 2}}
hdolhaine
(Hans Dolhaine)
April 12, 2022, 7:58pm
3
And some more solutions
2^# + 1 & /@ {2, 6, 10, 14}
hdolhaine
(Hans Dolhaine)
April 12, 2022, 9:28pm
4
In fact every number
z[m_] := 2^(2 + 4 m ) + 1
for m element Integers is divided by 5 without rest (proof by induction).