login
The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 

Logo

Please make a donation to keep the OEIS running. We are now in our 56th year. In the past year we added 10000 new sequences and reached almost 9000 citations (which often say "discovered thanks to the OEIS").
Other ways to donate

Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A074851 Numbers k such that k and k+1 both have exactly 2 distinct prime factors. 11
14, 20, 21, 33, 34, 35, 38, 39, 44, 45, 50, 51, 54, 55, 56, 57, 62, 68, 74, 75, 76, 85, 86, 87, 91, 92, 93, 94, 95, 98, 99, 111, 115, 116, 117, 118, 122, 123, 133, 134, 135, 141, 142, 143, 144, 145, 146, 147, 152, 158, 159, 160, 161, 171, 175, 176, 177, 183, 184 (list; graph; refs; listen; history; text; internal format)
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

Cf. A006049, A006549.

Analogous sequences for m distinct prime factors: this sequence (m=2), A140077 (m=3), A140078 (m=4), A140079 (m=5).

Cf. A093548.

Equals A255346 \ A321502.

Sequence in context: A006576 A083247 A255346 * A193672 A087678 A144585

Adjacent sequences:  A074848 A074849 A074850 * A074852 A074853 A074854

KEYWORD

easy,nonn

AUTHOR

Benoit Cloitre, Sep 10 2002

STATUS

approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified December 5 03:31 EST 2020. Contains 338943 sequences. (Running on oeis4.)