

A253073


Lexicographically earliest sequence of distinct numbers such that neither a(n) nor a(n1)+a(n) is prime.


3



0, 1, 8, 4, 6, 9, 12, 10, 14, 16, 18, 15, 20, 22, 24, 21, 25, 26, 28, 27, 30, 32, 33, 35, 34, 36, 38, 39, 42, 40, 44, 46, 45, 48, 50, 49, 51, 54, 52, 56, 55, 57, 58, 60, 62, 63, 65, 64, 66, 68, 70, 72, 69, 74, 76, 77, 75, 78, 80, 81, 84, 82, 86, 85, 87, 88
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OFFSET

1,3


COMMENTS

Conjecture: this is a permutation of the nonprimes. [Proof outline given below by Semeon Artamonov and Pat Devlin.]
Let x be a number that's missing.
Then eventually every term must be of the form PRIME  x. (Otherwise, x would appear as that next term.)
In particular, this means there are only finitely many multiples of x that appear in the sequence. To make this cleaner, let Y be a multiple of x larger than all multiples of x appearing in the sequence.
Let q be a prime not dividing Y. Then since none of the terms Y, 2Y, 3Y, ..., 2qY appear, it must be that, eventually, every term in the sequence is of the form PRIME  Y and also of the form PRIME  2Y and also of the form PRIME  3Y, ... and also of the form PRIME  2qY.
That means we have a prime p and a number Y such that p, p+Y, p+2Y, p + 3Y, p+4Y, ..., p+2qY are all prime. But take this sequence mod q. Since q does not divide Y, the terms 0, Y, ..., 2qY cover every residue class mod q twice. Therefore, p + kY covers each residue class mod q twice. Consequently, there are two terms congruent to 0 mod q. One can be q, but the other must be a multiple of it (contradicting its primality).


LINKS

Reinhard Zumkeller, Table of n, a(n) for n = 1..10000


PROG

(Haskell)
a253073 n = a253073_list !! (n1)
a253073_list = 0 : f 0 a018252_list where
f u vs = g vs where
g (w:ws)  a010051' (u + w) == 1 = g ws
 otherwise = w : f w (delete w vs)
 Reinhard Zumkeller, Feb 02 2015


CROSSREFS

Cf. A254337, A002808, A253074.
Cf. A018252, A010051.
Sequence in context: A255987 A266556 A199434 * A090325 A090469 A322743
Adjacent sequences: A253070 A253071 A253072 * A253074 A253075 A253076


KEYWORD

nonn


AUTHOR

N. J. A. Sloane, Feb 01 2015, based on a suggestion from Patrick Devlin


STATUS

approved



