A376734 - OEIS

%I #17 Nov 04 2024 02:07:39

%S 332,655,773,943,1007,1018,1033,1046,1117,1172,1277,1333,1358,1369,

%T 1424,1622,1667,1783,1810,1828,1865,1907,1928,2008,2216,2252,2293,

%U 2348,2404,2447,2473,2518,2567,2608,2645,2698,2711,2726,2797,2898,2942,2972,2978,3031,3048,3049

%N Numbers k such that 2*k-1 and 2*k+1 are products of exactly three distinct odd primes (A046389).

%H Dominic McCarty, <a href="/A376734/b376734.txt">Table of n, a(n) for n = 1..10000</a>

%p q:= k-> andmap(x-> map(i-> i[2], ifactors(x)[2])=[1$3], [2*k-1, 2*k+1]):

%p select(q, [$1..4000])[];  # _Alois P. Heinz_, Oct 18 2024

%o (PARI) is_a046389(k) = k%2 && omega(k)==3 && bigomega(k)==3;

%o is_a376734(n) = is_a046389(2*n-1) && is_a046389(2*n+1)

%Y Cf. A046389, A234099, A234102.

%K nonn

%O 1,1

%A _Hugo Pfoertner_, Oct 18 2024