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.

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

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 * A354489 A284371 A123438

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