This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A381189 #10 Mar 02 2025 23:54:43
%S A381189 6,26,38,57,62,69,77,82,87,106,145,155,177,206,209,219,221,253,309,
%T A381189 319,339,341,358,382,451,485,497,502,566,685,695,734,781,849,866,893,
%U A381189 905,949,1018,1037,1079,1081,1101,1157,1167,1169,1186,1191,1257,1313,1355,1387,1389
%N A381189 Ulam numbers that are squarefree semiprimes.
%C A381189 Ulam numbers: a(1) = 1; a(2) = 2; for n>2, a(n) = least number > a(n-1) which is a unique sum of two distinct earlier terms.
%C A381189 Number such as 4, 2809, 3481, 6889, etc., are in A378795 but not in this or in a longer sequence.
%e A381189 4 is not a term of this sequence because 4=2^2 is the product of 2 not distinct primes even if 4 is an Ulam number.
%e A381189 6 is a term because 6=2*3 is the product of 2 distinct primes and 6 is an Ulam number.
%e A381189 57 is a term because 57=3*19 is the product of 2 distinct primes and 57 is an Ulam number.
%e A381189 2809 is not a term of a longer sequence because 2809=53^2 is the product of 2 not distinct primes even if 2809 is an Ulam number.
%t A381189 seq[numUlams_] := Module[{ulams = {1, 2}}, Do[AppendTo[ulams, n = Last[ulams]; While[n++; Length[DeleteCases[Intersection[ulams, n - ulams], n/2, 1, 1]] != 2]; n], {numUlams}]; Select[ulams, FactorInteger[#][[;; , 2]] == {1, 1} &]]; seq[160] (* _Amiram Eldar_, Feb 16 2025, after _Jean-François Alcover_ at A002858 *)
%Y A381189 Intersection of A006881 and A002858.
%Y A381189 Cf. A068820, A378795, A379162, A379532.
%K A381189 nonn
%O A381189 1,1
%A A381189 _Massimo Kofler_, Feb 16 2025