A140494 - OEIS

A140494

Numbers that cannot be part of the trajectory of any number under repeated application of the map: n -> n + third-smallest number that does not divide n, unless they are the first term of the trajectory.

1, 2, 3, 4, 6, 12, 18, 24, 28, 30, 36, 37, 40, 42, 46, 48, 49, 52, 54, 55, 60, 64, 66, 72, 78, 80, 84, 88, 90, 96, 97, 100, 102, 106, 108, 112, 114, 115, 120, 124, 126, 132, 133, 138, 144, 150, 156, 157, 160, 162, 166, 168, 172, 174, 175, 180, 184, 186, 192, 198, 200

(list; graph; refs; listen; history; text; internal format)

OFFSET

1,2

LINKS

Table of n, a(n) for n=1..61.

PROG

(PARI) third(n) = {my(nb = 0, k = 1); while (nb != 3, if (n % k, nb++); if (nb != 3, k++); ); k; }

f(n) = n + third(n);

canbe(n) = {for (k=1, n, if (k + third(k) == n, return (1)); ); return (0); }

cannotbe(n) = 1 - canbe(n);

lista(nn) = {for (n=1, nn, if (cannotbe(n), print1(n, ", ")); ); } \\ Michel Marcus, Oct 04 2018

CROSSREFS