|
%I #5 Jun 17 2021 18:11:27
%S 5,7,11,13,17,19,31,37,41,43,53,59,61,71,73,79,83,89,97,103,109,113,
%T 131,137,139,149,151,157,163,167,173,179,181,191,193,197,199,211,223,
%U 227,229,233,251,257,263,269,271,281,293,307,313,317,337,347,349,353,359
%N Primes of the form c(c(i)-3)-3, where A002808(i)=c(i)=i-th composite.
%e If i=1, then
%e c(c(1)-3)-3=c(4-3)-3=4-3=1 (nonprime).
%e If i=2, then
%e c(c(2)-3)-3=c(6-3)-3=c(3)-3=8-3=5=a(1).
%e If i=3, then
%e c(c(3)-3)-3=c(8-3)-3=c(5)-3=10-3=7=a(2).
%e If i=4, then
%e c(c(4)-3)-3=c(9-3)-3=c(6)-3=12-3=9 (nonprime).
%e If i=5, then
%e c(c(5)-3)-3=c(10-3)-3=c(7)-3=14-3=11=a(3),
%e etc.
%t Module[{nn=500,cmps},cmps=Select[Range[nn],CompositeQ];Select[ Table[ cmps[[cmps[[n]]-3]]-3,{n,250}],PrimeQ]] (* _Harvey P. Dale_, Jun 17 2021 *)
%Y Cf. A002808, A000040.
%K nonn
%O 1,1
%A _Juri-Stepan Gerasimov_, Sep 03 2008
%E 43, 59, 73, 227, 229, 347, 367 inserted by _R. J. Mathar_, Sep 05 2008
|
