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A309944 Numbers m such that if m = p_1^e_1 * ... * p_k^e_k, where p_1 < ... < p_k are primes, then for all i < k, p_i = A000720(p_{i+1}). 0

%I #26 Sep 08 2022 08:46:22

%S 6,12,15,18,24,30,36,45,48,54,55,60,72,75,90,96,108,119,120,135,144,

%T 150,162,165,180,192,216,225,240,270,275,288,300,324,330,341,360,375,

%U 384,405,432,450,480,486,495,533,540,576,600,605,648,660,675,720,750,768

%N Numbers m such that if m = p_1^e_1 * ... * p_k^e_k, where p_1 < ... < p_k are primes, then for all i < k, p_i = A000720(p_{i+1}).

%C Numbers m such that for all k, d(k) = prime(d(k-1)), where d(k) is the k-th prime factor of m.

%C The primitive subsequence b(k), k = 1, 2, ... begins with 6, 15, 30, 55, 110, 165, 330, 341, 533, ... because if d(i) is the i-th prime factor of b(k), so b(k)*d(i)^m is in the sequence, m = 0, 1, 2, ...

%C Numbers m such that if m = p_1^e_1 * ... * p_k^e_k, p_1 < ... < p_k primes, then for all i > 1, p_i = A000040(p_{i-1}). - _Antti Karttunen_, Aug 24 2019

%H <a href="/index/Pri#prime_indices">Index entries for sequences computed from indices in prime factorization</a>

%e 330 is in the sequence because the prime factors are {2, 3, 5, 11} with 3 = prime(2), 5 = prime(3) and 11 = prime(5).

%e 1299210 is in the sequence because the prime factors are {2, 3, 5, 11, 31, 127} with 3 = prime(2), 5 = prime(3), 11 = prime(5), 31 = prime(11) and 127 = prime(31).

%p with(numtheory):nn:=10^3:

%p for n from 1 to nn do:

%p d:=factorset(n):n0:=nops(d):it:=0:

%p if n0>1

%p then

%p for i from 2 to n0 do :

%p if d[i]=ithprime(d[i-1])

%p then

%p it:=it+1:

%p else fi:

%p od:

%p if it=n0-1

%p then

%p printf(`%d, `,n):

%p else fi:fi:

%p od:

%t aQ[n_] := (m = Length[(p = FactorInteger[n][[;; , 1]])]) > 1 && NestList[Prime@# &, p[[1]], m - 1] == p; Select[Range[770], aQ] (* _Amiram Eldar_, Aug 24 2019 *)

%o (Magma) sol:=[]; s:=1; for m in [2..1000] do v:=PrimeDivisors(m); if #v ge 2 then nr:=0; for k in [2..#v] do if v[k] eq NthPrime(v[k-1]) then nr:=nr+1; end if; end for; if nr eq #v-1 then sol[s]:=m;s:=s+1; end if; end if; end for; sol; // _Marius A. Burtea_, Aug 24 2019

%o (PARI) isok(m) = {my(f=factor(m)[,1]~); if (#f < 2, return(0)); for (i=2, #f, if (f[i] != prime(f[i-1]), return (0));); return (1);} \\ _Michel Marcus_, Aug 25 2019

%Y Cf. A000040, A000720, A006450, A027746, A027748

%Y Contains A033845, A033849, A143207.

%K nonn

%O 1,1

%A _Michel Lagneau_, Aug 24 2019

%E Edited by _N. J. A. Sloane_, Oct 05 2019, using definition suggested by _Antti Karttunen_, Aug 24 2019

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Last modified March 19 01:57 EDT 2024. Contains 370952 sequences. (Running on oeis4.)