A050438 - OEIS

%I #7 Oct 27 2023 22:00:44

%S 26,33,38,39,42,49,52,55,56,60,68,69,70,74,77,78,80,84,88,93,94,95,98,

%T 100,105,106,110,115,118,119,121,124,125,126,130,133,138,140,141,145,

%U 146,152,154,155,156,159,160,162,164,165,170,174,176,180,183,184

%N Fourth-order composites.

%H N. Fernandez, <a href="http://www.borve.org/primeness/FOP.html">An order of primeness, F(p)</a>

%H N. Fernandez, <a href="/A006450/a006450.html">An order of primeness</a> [cached copy, included with permission of the author]

%F Let C(n) be the n-th composite number, with C(1)=4. Then these are numbers C(C(C(C(n)))).

%e C(C(C(C(8)))) = C(C(C(15))) = C(C(25)) = C(38) = 55. So 55 is in the sequence.

%p C := remove(isprime,[$4..1000]): seq(C[C[C[C[n]]]],n=1..100);

%Y Cf. A049076-A049081, A006450, A050435, A050436, A050439, A050440.

%K easy,nonn

%O 1,1

%A Michael Lugo (mlugo(AT)thelabelguy.com), Dec 22 1999

%E More terms from _Asher Auel_ Dec 15 2000