A337801 Numbers of the form (2^i)*(5^j) or (3^i)*(5^j).

1, 2, 3, 4, 5, 8, 9, 10, 15, 16, 20, 25, 27, 32, 40, 45, 50, 64, 75, 80, 81, 100, 125, 128, 135, 160, 200, 225, 243, 250, 256, 320, 375, 400, 405, 500, 512, 625, 640, 675, 729, 800, 1000, 1024, 1125, 1215, 1250, 1280, 1600, 1875, 2000, 2025, 2048, 2187, 2500

Offset: 1

Author

Vaclav Kotesovec, Sep 22 2020

Keywords

• nonn
• easy

Comments

Union of A003592 and A003593.

Links

• Vaclav Kotesovec, Table of n, a(n) for n = 1..10000
• Vaclav Kotesovec, Graph - the asymptotic ratio (42000000 terms)

Crossrefs

Cf. A003592, A003593, A257997, A258023, A337800.

Programs

Mathematica

n = 10^4; Join[Table[2^i*5^j, {i, 0, Log[2, n]}, {j, 0, Log[5, n/2^i]}], Table[3^i*5^j, {i, 0, Log[3, n]}, {j, 0, Log[5, n/3^i]}]] // Flatten // Union (* Amiram Eldar, Sep 23 2020 *)

Formula

a(n) ~ exp(sqrt(2*log(2)*log(3)*log(5)*n / log(6))) / sqrt(5).

Sum_{n>=1} 1/a(n) = 25/8. - Amiram Eldar, Sep 23 2020