cp's OEIS Frontend

A115957 Numbers k having exactly 3 distinct prime factors, the largest of which is greater than or equal to sqrt(k) (i.e., sqrt(k)-rough numbers with exactly 3 distinct prime factors).

%I A115957 #13 Aug 04 2025 20:53:11
%S A115957 42,66,78,102,110,114,130,138,156,170,174,186,190,204,222,228,230,238,
%T A115957 246,255,258,266,276,282,285,290,310,318,322,342,345,348,354,366,370,
%U A115957 372,402,406,410,414,426,430,434,435,438,444,460,465,470,474,483,492
%N A115957 Numbers k having exactly 3 distinct prime factors, the largest of which is greater than or equal to sqrt(k) (i.e., sqrt(k)-rough numbers with exactly 3 distinct prime factors).
%H A115957 Robert Israel, <a href="/A115957/b115957.txt">Table of n, a(n) for n = 1..10000</a>
%e A115957 156 is in the sequence because it has 3 distinct prime factors (2, 3 and 13) and 13 > sqrt(156).
%p A115957 with(numtheory): a:=proc(n) if nops(factorset(n))=3 and factorset(n)[3]^2>=n then n else fi end: seq(a(n),n=1..530);
%t A115957 Select[Range[500],PrimeNu[#]==3&&FactorInteger[#][[-1,1]]>=Sqrt[#]&] (* _Harvey P. Dale_, Apr 09 2019 *)
%Y A115957 Cf. A115956, A115958, A115959, A115960, A115961.
%K A115957 nonn
%O A115957 1,1
%A A115957 _Emeric Deutsch_, Feb 02 2006