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A074987
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a(n) is the least m not equal to n such that phi(m) = phi(n).
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2
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2, 1, 4, 3, 8, 3, 9, 5, 7, 5, 22, 5, 21, 7, 16, 15, 32, 7, 27, 15, 13, 11, 46, 15, 33, 13, 19, 13, 58, 15, 62, 17, 25, 17, 39, 13, 57, 19, 35, 17, 55, 13, 49, 25, 35, 23, 94, 17, 43, 25, 64, 35, 106, 19, 41, 35, 37, 29, 118, 17, 77, 31, 37, 51, 104, 25, 134, 51, 92, 35, 142
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OFFSET
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1,1
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COMMENTS
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In 1922, Carmichael asked if for any given natural number n there exists a natural number m different from n such that phi(m) = phi(n). A. Schlafly and S. Wagon showed in 1994 that if there is an n such that phi(m) != phi(n) for all m distinct from n, then n must be greater than 10^(10^7). [Improved to 10^(10^10) by Kevin Ford. - Pontus von Brömssen, May 15 2020]
I conjecture that a(n) <= 2n. I have checked this for all n <= 10^4. (It is not possible to do better than the 2n upper bound since a(11) = 2*11.)
For odd n the conjecture is true because phi(n)=phi(2n). - T. D. Noe, Oct 18 2006
If a(n) > n then a(a(n)) = n.
If n is in A138537 then a(n) = 2*n. (End)
Its complement, A296214, has values n such that a(n) < n. (End)
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REFERENCES
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J. Tattersall, "Elementary Number Theory in Nine Chapters", Cambridge University Press, 2001, pp. 162-163.
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LINKS
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EXAMPLE
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phi(5) = 4 and 8 is the least natural number k different from 5 such phi(k) = 4. Hence phi(5) = 8.
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MAPLE
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N:= 1000: # to get a(n) for n <= N
todo:= N;
for n from 1 while todo > 0 do
v:= numtheory:-phi(n);
if assigned(R[v]) then
if n <= N then
A[n]:= R[v]; todo:= todo-1;
fi;
if R[v] <= N and not assigned(A[R[v]]) then
A[R[v]]:= n; todo:= todo-1;
fi;
else
R[v]:= n
fi
od:
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MATHEMATICA
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l = {}; Do[ e = EulerPhi[n]; i = 1; While[e != EulerPhi[i] || n == i, i++ ]; l = Append[l, i], {n, 1, 100}]; l
(* Second program: *)
Module[{nn=300, lst}, lst=Table[{n, EulerPhi[n]}, {n, nn}]; Take[Table[ SelectFirst[ lst, #[[2]] == lst[[k, 2]] && #[[1]]!=lst[[k, 1]]&], {k, nn}], 100]][[All, 1]] (* Requires Mathematica version 10 or later *) (* Harvey P. Dale, Oct 23 2020 *)
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PROG
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(Python)
from sympy import totient
m=1
while totient(m)!=totient(n) or m==n:
m+=1
(PARI) a(n) = my(t=eulerphi(n), m=1); while ((eulerphi(m) != t) || (m==n), m++); m; \\ Michel Marcus, May 15 2020
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CROSSREFS
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KEYWORD
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easy,nice,nonn
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AUTHOR
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STATUS
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approved
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