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 A263981 Least even k such that phi(k) >= n. 0
 2, 4, 8, 8, 14, 14, 16, 16, 22, 22, 26, 26, 32, 32, 32, 32, 38, 38, 44, 44, 46, 46, 52, 52, 58, 58, 58, 58, 62, 62, 64, 64, 74, 74, 74, 74, 82, 82, 82, 82, 86, 86, 92, 92, 94, 94, 104, 104, 106, 106, 106, 106, 116, 116, 116, 116, 118, 118, 122, 122, 128, 128 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Representation number of the bipartite graph K_{1,n} (the n-pointed star graph), see Akhtar, Evans, & Pritikin. A graph G is said to have a representation mod r if each of its vertices can be given a unique label mod r such that two vertices are adjacent if and only if the difference of their representation numbers is coprime to r. The representation number of G is the least r for which G has a representation mod r, see Erdős & Evans. LINKS Reza Akhtar, Anthony B. Evans, and Dan Pritikin, Representation number of stars, Integers 10 (2010), pp. 733-745. #A54 P. Erdős and A. B. Evans, Representations of graphs and orthogonal Latin square graphs, J. Graph Theory 13:5 (1989), pp. 593-595. FORMULA 2n <= a(n) <= 2*A151800(n). EXAMPLE The star graph with center C and other points P1, P2, P3 can be labeled with C = 0 mod 8, P1 = 1 mod 8, P2 = 3 mod 8, and P3 = 5 mod 8 so that two points are adjacent iff their difference is odd (=coprime to 8), so a(3) <= 8. MATHEMATICA Table[k = 2; While[EulerPhi@ k < n, k += 2]; k, {n, 62}] (* Michael De Vlieger, Nov 16 2015 *) PROG (PARI) /* oo = 10^10; */ /* uncomment for earlier pari versions */ a(n)=forstep(k=2*n, oo, 2, if(eulerphi(k)>=n, return(k))) \\ Charles R Greathouse IV, Oct 30 2015 (PARI) a(n)=my(k=2*n); while(eulerphi(k)

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Last modified February 23 04:44 EST 2019. Contains 320411 sequences. (Running on oeis4.)