A066721 - OEIS

%I #18 Jun 15 2018 21:26:57

%S 1,8,18,50,60,81,98,105,128,242,264,308,338,416,495,520,546,560,578,

%T 625,663,675,684,864,935,952,1029,1058,1083,1224,1242,1254,1425,1430,

%U 1682,1729,1748,1771,1827,1922,2436,2691,2697,2720,2738,2755,2790,2975

%N Nonprimes in A066720.

%C 2*p^2 is in the sequence for all primes except those in A066775.

%H David Applegate, <a href="/A066721/b066721.txt">First 48186 terms of A066721</a>

%H David Applegate, <a href="/A066721/a066721p.txt">C program for computing this sequence</a>

%H David Applegate, <a href="/A066721/a066721.txt">First 48186 terms of A066721 and their factorizations</a>

%t s={1}; xok := Module[ {}, For[ i=1, i<=n, i++, For[ j=1; k=Length[ dl=Divisors[ s[[ i ]]x ]], j<=k, j++; k--, If[ MemberQ[ s, dl[[ j ] ]]&&MemberQ[ s, dl[[ k ] ]], Return[ False ]] ]]; True ]; For[ n=1, True, n++, If[ !PrimeQ[ s[[ n ] ]], Print[ s[[ n ]] ]]; For[ x=s[[ n ]]+1, True, x++, If[ xok, AppendTo[ s, x ]; Break[ ]] ]]

%o (PARI) {a066721(m) = local(a,rat,n,s,new,b,i,k,j); a=[]; rat=Set([]); n=0; s=0; while(s<m,s++; new=Set([]); b=1; i=1; while(b&&i<=n,k=s/a[i]; if(setsearch(rat,k),b=0,new=setunion(new,Set(k)); k=a[i]/s; if(setsearch(rat,k),b=0,new=setunion(new,Set(k)) )); i++); if(b,rat=setunion(rat,new); a=concat(a,s); n++; if(!isprime(s),print1(s,","))))} a066721(3000) \\ _Klaus Brockhaus_, Feb 23 2002

%o (Haskell)

%o a066721 n = a066721_list !! (n-1)

%o a066721_list = filter ((== 0) . a010051') a066720_list

%o -- _Reinhard Zumkeller_, Nov 19 2013

%Y Cf. A066720, A066775.

%Y Cf. A010051.

%K nonn,nice

%O 1,2

%A _N. J. A. Sloane_, Jan 15 2002

%E More terms from _Dean Hickerson_, _Klaus Brockhaus_ and _David Applegate_, Jan 15 2002