OFFSET
1,1
COMMENTS
Subsequence of A006049. - Michel Marcus, May 06 2016
LINKS
Seiichi Manyama, Table of n, a(n) for n = 1..10000
FORMULA
a(n) seems to be asymptotic to c*n*log(n)^2 with c=0.13...
EXAMPLE
20=2^2*5 21=3*7 hence 20 is in the sequence.
MATHEMATICA
Flatten[Position[Partition[Table[If[PrimeNu[n]==2, 1, 0], {n, 200}], 2, 1], {1, 1}]] (* Harvey P. Dale, Mar 12 2015 *)
PROG
(PARI) isok(n) = (omega(n) == 2) && (omega(n+1) == 2); \\ Michel Marcus, May 06 2016
(Magma) [n: n in [2..200] | #PrimeDivisors(n) eq 2 and #PrimeDivisors(n+1) eq 2]; // Vincenzo Librandi, Dec 05 2018
(GAP) Filtered([1..200], n->[Size(Set(Factors(n))), Size(Set(Factors(n+1)))]=[2, 2]); # Muniru A Asiru, Dec 05 2018
(Python)
import sympy
from sympy.ntheory.factor_ import primenu
for n in range(1, 200):
if primenu(n)==2 and primenu(n+1)==2:
print(n, end=', '); # Stefano Spezia, Dec 05 2018
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Benoit Cloitre, Sep 10 2002
STATUS
approved