A392987 The number of 2-dense numbers <= n where the 2-dense numbers are A174973.

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, 12, 13, 13, 13, 13, 14, 14, 14, 14, 15, 15, 16, 16, 16, 16, 16, 16, 17, 17, 17, 17, 17, 17, 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

Offset

1,2

Comments

First differs from A225559 at a(78).

Links

Antti Karttunen, Table of n, a(n) for n = 1..10000

Formula

A174973(a(n)) = n.

Mathematica

Accumulate[Array[Boole[Max[Rest[#]/Most[#]] <= 2] & [Divisors[#]] &, 100]] (* Paolo Xausa, Mar 13 2026 *)

Prog

(PARI)
up_to = 10000;
A365429(n) = { my(d=divisors(n)); for(i=2, #d, if(d[i]>2*d[i-1], return(0))); (1); };
A392987_list(up_to) = { my(v=vector(up_to), s=0); for(i=1, up_to, s+=A365429(i); v[i] = s); (v); };
v392987 = A392987_list(up_to);
A392987(n) = v392987[n]; \\ Antti Karttunen, Mar 05 2026

Crossrefs

Partial sums of A365429.

Cf. A174973, A225559.

Sequence in context: A219652 A069924 A377028 * A225559 A383278 A082479

Adjacent sequences: A392984 A392985 A392986 * A392988 A392989 A392990

Keyword

nonn

Author

Omar E. Pol, Mar 05 2026

Status

approved