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 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

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Last modified February 21 23:29 EST 2019. Contains 320381 sequences. (Running on oeis4.)