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Largest non-isolated nonprime < n-th non-isolated (or twin) prime.
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%I #8 Feb 18 2019 11:11:38

%S 1,1,1,10,10,16,16,28,28,40,40,58,58,70,70,100,100,106,106,136,136,

%T 148,148,178,178,190,190,196,196,226,226,238,238,268,268,280,280,310,

%U 310,346,346,418,418,430,430,460,460,520,520,568,568,598,598,616,616,640

%N Largest non-isolated nonprime < n-th non-isolated (or twin) prime.

%e a(1)=1 (<3); a(2)=1 (<5); a(3)=1 (<7); a(4)=10 (<11).

%p isA164276 := proc(n) not isprime(n) and ( not isprime(n+1) or not isprime(n-1) ) ; end proc:

%p isA001097 := proc(n) isprime(n) and ( isprime(n+2) or isprime(n-2) ); end proc:

%p A001097 := proc(n) option remember; if n =1 then 3; else for a from procname(n-1)+2 by 2 do if isA001097(a) then return a; end if; end do: end if; end proc:

%p A167427 := proc(n) for a from A001097(n)-1 by -1 do if isA164276(a) then return a; end if; end do: end proc:

%p seq(A167427(n),n=1..120) ; # _R. J. Mathar_, May 30 2010

%Y Cf. A001097, A164276.

%K nonn

%O 1,4

%A _Juri-Stepan Gerasimov_, Nov 03 2009

%E Entries checked by _R. J. Mathar_, May 30 2010