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A264780 a(n) is the index of A246947(n) in A000977.

%I A264780 #9 Nov 26 2015 03:39:54
%S A264780 1,3,8,13,19,26,33,2,7,14,23,43,57,67,81,93,5,18,52,72,115,138,164,10,
%T A264780 63,127,200,240,41,49,58,66,4,16,31,47,85,107,20,60,159,214,11,34,100,
%U A264780 134,175,22,118,234,296,75,84,6,21,39,62,109,133,27,76,197,265
%N A264780 a(n) is the index of A246947(n) in A000977.
%C A264780 Is this a permutation of the positive integers?
%H A264780 Robert Israel, <a href="/A264780/b264780.txt">Table of n, a(n) for n = 1..10000</a>
%e A264780 The first 5 terms of A246947 are 30, 60, 90, 120, and 150, that is, the 1st, 3rd, 8th, 13th and 19th terms of A000977.
%p A264780 N:= 1000: # get all terms before the first term > N
%p A264780 W:= Vector(N, t -> if nops(numtheory:-factorset(t))<=2 then 0 else 1 fi):
%p A264780 WS:= ListTools:-PartialSums(convert(W, list)):
%p A264780 m:= 1:
%p A264780 F:= {2, 3,5}:
%p A264780 A[1]:= WS[30]:
%p A264780 W[30]:= 0:
%p A264780 for n from 2 do
%p A264780     while W[m] = 0 and m < N do m:= m+1 od;
%p A264780   for k from m to N do
%p A264780      if W[k] = 1 and nops(numtheory:-factorset(k) intersect F) = 3 then
%p A264780         A[n]:= WS[k];
%p A264780         W[k]:= 0;
%p A264780         F:= numtheory:-factorset(k);
%p A264780         break
%p A264780      fi
%p A264780   od;
%p A264780   if k > N then break fi;
%p A264780 od:
%p A264780 seq(A[i],i=1..n-1); # _Robert Israel_, Nov 26 2015
%o A264780 (PARI) v246947(nn) = {a = 30; fa = (factor(a)[,1])~; va = [a]; vs = va; k = 0; while (k < nn, k = 1; while (!((#setintersect(fa, (factor(k)[,1])~) == 3) && (! vecsearch(vs, k))), k++); a = k; fa = (factor(a)[,1])~; va = concat(va, k); vs = vecsort(va);); va;}
%o A264780 v000977(nn) = {va = []; for (n=1, nn, if (omega(n) >= 3, va = concat(va, n));); va;}
%o A264780 lista(nn) = {v = v246947(nn); w = v000977(vecmax(v)); for (k=1, #v, for (j=1, #w, if (w[j] == v[k], print1(j, ", "); break);););}
%Y A264780 Cf. A000977, A246947, A264762.
%K A264780 nonn
%O A264780 1,2
%A A264780 _Michel Marcus_, Nov 24 2015