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A249053 Defined by (i) a(1)=1; (ii) if you move a(n) steps to the right you must reach a prime; (iii) a(n) = smallest unused composite number greater than a(n-1), unless a(n) is required to be prime by (ii), in which case a(n) is the smallest unused prime greater than a(n-1). 2

%I #11 Nov 01 2014 06:07:15

%S 1,2,4,5,6,8,11,12,13,14,17,18,20,23,24,25,26,29,30,31,32,37,38,41,42,

%T 44,45,47,48,53,54,55,59,60,62,63,67,68,71,72,73,74,79,80,81,82,83,84,

%U 89,90,97,98,101,102,104,105,106,108,109,110,113,114,115

%N Defined by (i) a(1)=1; (ii) if you move a(n) steps to the right you must reach a prime; (iii) a(n) = smallest unused composite number greater than a(n-1), unless a(n) is required to be prime by (ii), in which case a(n) is the smallest unused prime greater than a(n-1).

%D Eric Angelini, Posting to Sequence Fans Mailing List, Mar 17 2008 (the definition was clarified by Gabriel Cunningham).

%H Reinhard Zumkeller, <a href="/A249053/b249053.txt">Table of n, a(n) for n = 1..10000</a>

%o (Haskell)

%o import Data.Map (singleton, findMin, delete, insert)

%o a249053 n = a249053_list !! (n-1)

%o a249053_list = 1 : f 1 1 a002808_list (singleton 1 1) where

%o f x z cs m

%o | k == x = p : f (x + 1) p cs (insert (x + p) 0 $ delete x m)

%o | otherwise = c : f (x + 1) c cs' (insert (x + c) 0 m)

%o where p = a007918 z

%o (c:cs') = dropWhile (<= z) cs

%o (k,_) = findMin m

%o -- _Reinhard Zumkeller_, Nov 01 2014

%Y See A249054 for another version.

%Y Cf. A002808, A007918.

%K nonn

%O 1,2

%A _N. J. A. Sloane_, Nov 01 2014

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Last modified March 28 18:04 EDT 2024. Contains 371254 sequences. (Running on oeis4.)