A036304 Composite numbers whose prime factors contain no digits other than 1 and 4.

121, 451, 1331, 1681, 4961, 14641, 18491, 45221, 48851, 54571, 68921, 125521, 158521, 161051, 168551, 182081, 203401, 452551, 455521, 467851, 485221, 497431, 537361, 590851, 600281, 758131, 1380731, 1686781, 1697851, 1743731, 1771561

Offset: 1

Author

Patrick De Geest, Dec 15 1998

Keywords

• nonn
• easy
• base

Comments

All terms are a product of at least two terms of A020452. - David A. Corneth, Oct 09 2020

Examples

			The composite 4961 = 11^2 * 41 is in the sequence as the digits of its prime factors are either 1 or 4. - _David A. Corneth_, Oct 17 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.

Crossrefs

Cf. A020452, A036302-A036325.

Formula

Sum_{n>=1} 1/a(n) = Product_{p in A020452} (p/(p - 1)) - Sum_{p in A020452} 1/p - 1 = 0.0122909749... . - Amiram Eldar, May 18 2022