

A055265


a(n) is the smallest positive integer not already in the sequence such that a(n)+a(n1) is prime, starting with a(1)=1.


21



1, 2, 3, 4, 7, 6, 5, 8, 9, 10, 13, 16, 15, 14, 17, 12, 11, 18, 19, 22, 21, 20, 23, 24, 29, 30, 31, 28, 25, 34, 27, 26, 33, 38, 35, 32, 39, 40, 43, 36, 37, 42, 41, 48, 49, 52, 45, 44, 53, 50, 47, 54, 55, 46, 51, 56, 57, 70, 61, 66, 65, 62, 69, 58, 73, 64, 63, 68, 59, 72, 67, 60
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OFFSET

1,2


COMMENTS

The sequence is welldefined (the terms must alternate in parity, and by Dirichlet's theorem a(n+1) always exists).  N. J. A. Sloane, Mar 07 2017
Does every positive integer eventually occur?  Dmitry Kamenetsky, May 27 2009. Reply from Robert G. Wilson v, May 27 2009: The answer is almost certainly yes, on probabilistic grounds.
It appears that this is the limit of the rows of A051237. That those rows do approach a limit seems certain, and given that that limit exists, that this sequence is the limit seems even more likely, but no proof is known for either conjecture.  Robert G. Wilson v, Mar 31 2011, edited by Franklin T. AdamsWatters, Mar 17 2011


LINKS

Zak Seidov, Table of n, a(n) for n = 1..10000 (First 1000 terms from T. D. Noe)
N. J. A. Sloane, Table of n, a(n) for n = 1..100000 (computed using Orlovsky's Mma program)
Index entries for sequences that are permutations of the natural numbers


EXAMPLE

a(5)=7 because 1, 2, 3 and 4 have already been used and neither 4+5=9 nor 4+6=10 are prime while 4+7=11 is prime.


MAPLE

A055265 := proc(n)
local a, i, known ;
option remember;
if n =1 then
1;
else
for a from 1 do
known := false;
for i from 1 to n1 do
if procname(i) = a then
known := true;
break;
end if;
end do:
if not known and isprime(procname(n1)+a) then
return a;
end if;
end do:
end if;
end proc:
seq(A055265(n), n=1..100) ; # R. J. Mathar, Feb 25 2017


MATHEMATICA

f[s_List] := Block[{k = 1, a = s[[ 1]]}, While[ MemberQ[s, k]  ! PrimeQ[a + k], k++ ]; Append[s, k]]; Nest[f, {1}, 71] (* Robert G. Wilson v, May 27 2009 *)
q=2000; a={1}; z=Range[2, 2*q]; While[Length[z]>q1, k=1; While[!PrimeQ[z[[k]]+Last[a]], k++]; AppendTo[a, z[[k]]]; z=Delete[z, k]]; Print[a] (*200 times faster*) (* Vladimir Joseph Stephan Orlovsky, May 03 2011 *)


PROG

(HP 50G Calculator) << DUPDUP + 2 > N M L << { 1 } 1 N 1  FOR i L M FOR j DUP j POS NOT IF THEN j DUP 'L' STO M 'j' STO END NEXT OVER i GET SWAP WHILE DUP2 + DUP ISPRIME? NOT REPEAT DROP DO 1 + 3 PICK OVER POS NOT UNTIL END END ROT DROP2 + NEXT >> >> [From Gerald Hillier, Oct 28 2008]
(Haskell)
import Data.List (delete)
a055265 n = a055265_list !! (n1)
a055265_list = 1 : f 1 [2..] where
f x vs = g vs where
g (w:ws) = if a010051 (x + w) == 1
then w : f w (delete w vs) else g ws
 Reinhard Zumkeller, Feb 14 2013
(PARI) v=[1]; n=1; while(n<50, if(isprime(v[#v]+n)&&!vecsearch(vecsort(v), n), v=concat(v, n); n=0); n++); v \\ Derek Orr, Jun 01 2015


CROSSREFS

Inverse permutation: A117922; fixed points: A117925; A117923=a(a(n)).  Reinhard Zumkeller, Apr 03 2006
Cf. A036440, A051237, A051239, A055266, A088643.
Cf. A086527 (the primes a(n)+a(n1)).
Cf. A070942 (n's such that a(1..n) is a permutation of (1..n)).  Moshe Levin, Oct 19 2011
Cf. A010051.
See also A076990, A243625.
See A282695 for deviation from identity sequence.
Sequence in context: A267308 A264965 A266644 * A117922 A266643 A263273
Adjacent sequences: A055262 A055263 A055264 * A055266 A055267 A055268


KEYWORD

easy,nice,nonn


AUTHOR

Henry Bottomley, May 09 2000


EXTENSIONS

Corrected by Hans Havermann, Sep 24 2002


STATUS

approved



