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A253073 Lexicographically earliest sequence of distinct numbers such that neither a(n) nor a(n-1)+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 (list; graph; refs; listen; history; text; internal format)
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 !! (n-1)

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

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Last modified December 2 11:44 EST 2021. Contains 349440 sequences. (Running on oeis4.)