A392987 - OEIS

%I #36 Mar 14 2026 09:45:47

%S 1,2,2,3,3,4,4,5,5,5,5,6,6,6,6,7,7,8,8,9,9,9,9,10,10,10,10,11,11,12,

%T 12,13,13,13,13,14,14,14,14,15,15,16,16,16,16,16,16,17,17,17,17,17,17,

%U 18,18,19,19,19,19,20,20,20,20,21,21,22,22,22,22,22,22,23,23,23,23,23,23,23

%N The number of 2-dense numbers <= n where the 2-dense numbers are A174973.

%C First differs from A225559 at a(78).

%H Antti Karttunen, <a href="/A392987/b392987.txt">Table of n, a(n) for n = 1..10000</a>

%F A174973(a(n)) = n.

%t Accumulate[Array[Boole[Max[Rest[#]/Most[#]] <= 2] & [Divisors[#]] &, 100]] (* _Paolo Xausa_, Mar 13 2026 *)

%o (PARI)

%o up_to = 10000;

%o A365429(n) = { my(d=divisors(n)); for(i=2, #d, if(d[i]>2*d[i-1], return(0))); (1); };

%o A392987_list(up_to) = { my(v=vector(up_to), s=0); for(i=1,up_to,s+=A365429(i); v[i] = s); (v); };

%o v392987 = A392987_list(up_to);

%o A392987(n) = v392987[n]; \\ _Antti Karttunen_, Mar 05 2026

%Y Partial sums of A365429.

%Y Cf. A174973, A225559.

%K nonn

%O 1,2

%A _Omar E. Pol_, Mar 05 2026