OFFSET
1,1
COMMENTS
Numbers with prime signature {1,1,1,k} with k >= 2. - Robert Israel, Nov 09 2025
LINKS
Robert Israel, Table of n, a(n) for n = 1..10000
MAPLE
N:= 10^4: # for terms <= N
P:= select(isprime, [2, seq(i, i=3..N/(2^2*3*5), 2)]):
nP:= nops(P):
Res:= NULL:
for i1 from 1 to nP do
p1:= P[i1];
for i2 from i1+1 to nP do
p2:= P[i2];
if p1 * p2 * 3 * 4 > N then break fi;
for i3 from i2 + 1 to nP do
p3:= P[i3];
if p1 * p2 * p3 * 4 > N then break fi;
for i4 from 1 to nP do
if member(i4, {i1, i2, i3}) then next fi;
p4:= P[i4];
for n4 from 2 do
x:= p1 * p2 * p3 * p4^n4;
if x > N then break fi;
Res:= Res, x
od od od od od:
Res:= sort([Res]); # Robert Israel, Nov 09 2025
MATHEMATICA
q[n_] := DivisorSum[n, 1 &, PrimeOmega[#] == 2 &] == 7; Select[Range[2500], q] (* Amiram Eldar, Dec 28 2021 *)
PROG
(PARI) isok(k) = sumdiv(k, d, bigomega(d)==2) == 7; \\ Michel Marcus, Dec 28 2021
CROSSREFS
KEYWORD
nonn
AUTHOR
Wesley Ivan Hurt, Dec 27 2021
STATUS
approved
