A036305 Composite numbers whose prime factors contain no digits other than 1 and 5.

25, 55, 121, 125, 275, 605, 625, 755, 1331, 1375, 1661, 3025, 3125, 3775, 5755, 6655, 6875, 7555, 8305, 12661, 14641, 15125, 15625, 16621, 18271, 18875, 22801, 28775, 33275, 34375, 37775, 41525, 57755, 63305, 73205, 75625, 77555, 77755, 78125

Offset

1,1

Comments

All terms are a product of at least two terms of A020453. - David A. Corneth, Oct 09 2020

Links

David A. Corneth, Table of n, a(n) for n = 1..10000 (first 1000 terms from Alois P. Heinz)

Index entries for sequences related to prime factors.

Formula

Sum_{n>=1} 1/a(n) = Product_{p in A020453} (p/(p - 1)) - Sum_{p in A020453} 1/p - 1 = 0.0873463128... . - Amiram Eldar, May 18 2022

Crossrefs

Cf. A003598, A020453, A036302-A036325.

Sequence in context: A339729 A091214 A338009 * A370351 A257708 A266817

Adjacent sequences: A036302 A036303 A036304 * A036306 A036307 A036308

Keyword

nonn,easy,base

Author

Patrick De Geest, Dec 15 1998

Status

approved