A169643 - OEIS

%I #13 Oct 22 2024 20:14:51

%S 1,2,6,10,19,28,42,51,75,79,104,114,138,148,152,178,187,212,221,247,

%T 278,338,348,372,423,465,490,504,525,539,669,679,683,709,729,848,858,

%U 873,883,909,961,1028,1071,1080,1089,1104,1202,1221,1247,1251,1354,1363

%N Numbers n such that neither composite(n)-+1 is composite.

%C a(n) = A066246(A014574(n)). - _Reinhard Zumkeller_, Apr 06 2010

%H Robert Israel, <a href="/A169643/b169643.txt">Table of n, a(n) for n = 1..10000</a>

%e a(1)=1 because composite(1)-1=3=prime and composite(1)+1=5=prime.

%p Comp:= remove(isprime,[$4..2000]): nC:= nops(Comp):

%p select(t -> isprime(Comp[t]-1) and isprime(Comp[t]+1), [$1..nC]); # _Robert Israel_, Oct 21 2024

%t Position[Select[Range[2000],CompositeQ],_?(AllTrue[#+{1,-1},PrimeQ]&),1,Heads->False]// Flatten (* _Harvey P. Dale_, Jan 16 2024 *)

%Y Cf. A002808, A014574, A066246.

%K nonn

%O 1,2

%A _Juri-Stepan Gerasimov_, Apr 04 2010

%E Corrected by _Ray Chandler_, Apr 05 2010