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 A057600 a(n) is smallest number m such that (m = product of n distinct primes) and all divisors of m are lucky numbers. 1

%I #10 May 10 2020 06:25:39

%S 3,21,777,61383

%N a(n) is smallest number m such that (m = product of n distinct primes) and all divisors of m are lucky numbers.

%C a(5) > 2*10^7, if it exists. - Vit Planocka (planocka(AT)mistral.cz), Sep 26 2003

%C a(5) > 4*10^9, if it exists. - _Giovanni Resta_, May 10 2020

%e a(2) = 3*7 = 21: divisors of 21 are 1, 3, 7, 21, all of which are lucky numbers.

%t lst = Range[1, 2*10^7, 2]; i = 2; While[ i <= (len = Length@lst) && (k = lst[[i]]) <= len, lst = Drop[lst, {k, len, k}]; i++ ]; fQ[n_] := Block[{d = Rest@Divisors@n, k = 1, lmt = DivisorSigma[0, n]}, While[k < lmt && MemberQ[lst, d[[k]]], k++ ]; k == lmt]; t = Table[0, {10}]; Do[ If[ fQ@lst[[n]], a = Length@ FactorInteger@lst[[n]]; If[ t[[a]] == 0, t[[a]] = lst[[n]]; Print[ lst[[n]] ]]], {n, 2, Length@lst}] (* _Robert G. Wilson v_, May 12 2006 *)

%Y Cf. A000959.

%K nonn,more

%O 1,1

%A _Naohiro Nomoto_, Oct 06 2000

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Last modified June 23 22:09 EDT 2024. Contains 373661 sequences. (Running on oeis4.)