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 A074987 a(n) is the least m not equal to n such that phi(m) = phi(n). 2
 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 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS 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 From Robert Israel, Aug 12 2016: (Start) If a(n) > n then a(a(n)) = n. If n is in A138537 then a(n) = 2*n. (End) From David A. Corneth, May 12 2018: (Start) A210719 has values n such that a(n) > n, so a(A210719(n)) = n. Its complement, A296214, has values n such that a(n) < n. (End) REFERENCES J. Tattersall, "Elementary Number Theory in Nine Chapters", Cambridge University Press, 2001, pp. 162-163. LINKS T. D. Noe, Table of n, a(n) for n=1..10000 K. Ford, The distribution of totients, arXiv:1104.3264 [math.NT], 2011. A. Schlafly and S. Wagon, Carmichael's conjecture on the Euler function is valid below 10^{10,000,000}, Mathematics of Computation, 63 No. 207(1994), 415-419. Wikipedia, Carmichael's_totient_function_conjecture EXAMPLE phi(5) = 4 and 8 is the least natural number k different from 5 such phi(k) = 4. Hence phi(5) = 8. MAPLE 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: seq(A[n], n=1..N); # Robert Israel, Aug 12 2016 MATHEMATICA l = {}; Do[ e = EulerPhi[n]; i = 1; While[e != EulerPhi[i] || n == i, i++ ]; l = Append[l, i], {n, 1, 100}]; l Module[{nn=300, lst}, lst=Table[{n, EulerPhi[n]}, {n, nn}]; Take[Table[ SelectFirst[ lst, #[]==lst[[k, 2]]&&#[]!=lst[[k, 1]]&], {k, nn}], 100]][[All, 1]] (* Requires Mathematica version 10 or later *) (* Harvey P. Dale, Oct 23 2020 *) PROG (Python) from sympy import totient def A074987(n):   m=1   while totient(m)!=totient(n) or m==n:     m+=1   return m # Pontus von Brömssen, May 15 2020 (PARI) a(n) = my(t=eulerphi(n), m=1); while ((eulerphi(m) != t) || (m==n), m++); m; \\ Michel Marcus, May 15 2020 CROSSREFS Cf. A138537, A210719, A296214. Sequence in context: A087207 A179206 A331740 * A294096 A128280 A182177 Adjacent sequences:  A074984 A074985 A074986 * A074988 A074989 A074990 KEYWORD easy,nice,nonn AUTHOR Joseph L. Pe, Oct 02 2002 STATUS approved

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Last modified October 16 06:14 EDT 2021. Contains 348040 sequences. (Running on oeis4.)