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A172999 a(1)=1, a(2)=4; for n>=3, a(n) is the smallest number larger than a(n-1) such that, for every k<n, a(n) is relatively prime to a(k) iff n is relatively prime to k. 4
1, 4, 5, 6, 7, 10, 11, 12, 25, 28, 29, 30, 31, 44, 175, 178, 179, 180, 181, 182, 275, 348, 349, 360, 371, 372, 395, 396, 397, 420, 421, 422, 725, 1074, 1309, 1310, 1319, 1448, 2945, 2954, 2957, 2970, 2971, 3016, 3325, 4188, 4189, 4190, 4213, 4214, 4475, 4526 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

LINKS

Alois P. Heinz, Table of n, a(n) for n = 1..300

MAPLE

a:= proc(n) option remember;

     local ok, m, k;

     if n<3 then 3*n-2

   else for m from a(n-1)+1 do

          ok:= true;

          for k from 1 to n-1 do

            if igcd(n, k)=1 xor igcd(m, a(k))=1

               then ok:= false; break fi

          od;

          if ok then break fi

        od; m

     fi

    end:

seq (a(n), n=1..50);  # Alois P. Heinz, Nov 21 2010

MATHEMATICA

t={1, 4}; Do[nxt=t[[-1]]+1; While[CoprimeQ[n, Range[n-1]] != CoprimeQ[nxt, t], nxt++]; AppendTo[t, nxt], {n, 3, 50}]; t

CROSSREFS

Cf. A172980, A151976, A159559, A159560, A159615, A159619, A159629, A159698, A160217.

Sequence in context: A255137 A047260 A284790 * A284371 A123438 A039024

Adjacent sequences:  A172996 A172997 A172998 * A173000 A173001 A173002

KEYWORD

nonn

AUTHOR

Vladimir Shevelev, Nov 21 2010

EXTENSIONS

More terms from Alois P. Heinz, Nov 21 2010

STATUS

approved

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Last modified August 6 12:33 EDT 2020. Contains 336246 sequences. (Running on oeis4.)