A214522 List of words over {1,5} with equal numbers of 1's and 5's.

15, 51, 1155, 1515, 1551, 5115, 5151, 5511, 111555, 115155, 115515, 115551, 151155, 151515, 151551, 155115, 155151, 155511, 511155, 511515, 511551, 515115, 515151, 515511, 551115, 551151, 551511, 555111

Offset

1,1

Comments

This is to 5 as A214521 is to 4, as A214488 is to 3, and as A214218 is to 2. All of these, interpreted as decimal integers are divisible by 3, because each pair of "1" and "5" contributes a digital sum of 6, hence the total is divisible by 3.

The subsequence of semiprimes begins 15, 51, 15151551, 51115551, 51515511, 51555111, 55515111, 1115155551, 1115515551, 1151551551,..

Links

Table of n, a(n) for n=1..28.

Mathematica

Sort[FromDigits/@Flatten[Table[Permutations[PadRight[{}, 2n, {1, 5}]], {n, 3}], 1]] (* Harvey P. Dale, Aug 30 2016 *)

Crossrefs

Cf. A214218, A214488, A214521.

Sequence in context: A201054 A009962 A211563 * A118238 A015234 A295339

Adjacent sequences: A214519 A214520 A214521 * A214523 A214524 A214525

Keyword

nonn,base,easy

Author

Jonathan Vos Post, Jul 19 2012

Status

approved