A054774 Numbers of the form 2^i*5^j where i+j is odd.

2, 5, 8, 20, 32, 50, 80, 125, 128, 200, 320, 500, 512, 800, 1250, 1280, 2000, 2048, 3125, 3200, 5000, 5120, 8000, 8192, 12500, 12800, 20000, 20480, 31250, 32000, 32768, 50000, 51200, 78125, 80000, 81920, 125000, 128000, 131072, 200000

Offset: 1

Author

Henry Bottomley, May 19 2000

Keywords

• easy
• nonn

Comments

m is in the sequence iff m/10 is already in the sequence or m is an odd power of 2 or 5.

Links

• Amiram Eldar, Table of n, a(n) for n = 1..10000

Crossrefs

Intersection of A026424 and A003592.

Cf. A004171, A013710.

Programs

Mathematica

max = 200000; Reap[Do[k = 2^i*5^j; If[k <= max && OddQ[i + j], Sow[k]], {i, 0, Log[2, max] // Ceiling}, {j, 0, Log[5, max] // Ceiling}]][[2, 1]] // Union (* Amiram Eldar, Feb 18 2021 after Jean-François Alcover at A036667 *)

Formula

Sum_{n>=1} 1/a(n) = 35/36. - Amiram Eldar, Feb 18 2021