

A357074


Numbers sandwiched between a pair of numbers each with exactly two prime factors (counted without multiplicity).


1



11, 13, 19, 21, 23, 25, 27, 34, 35, 37, 39, 45, 47, 49, 51, 53, 55, 56, 57, 64, 73, 75, 76, 81, 86, 87, 92, 93, 94, 95, 97, 99, 105, 107, 116, 117, 118, 123, 134, 135, 142, 143, 144, 145, 146, 147, 154, 159, 160, 161, 163, 165, 176, 177, 184, 186, 188, 193, 195
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OFFSET

1,1


COMMENTS

Number k such that both k1 and k+1 are in A007774.


LINKS



EXAMPLE

11 is sandwiched between 10 = 2*5 and 12 = 2^2*3. Both 10 and 12 have exactly two prime factors. Thus, 11 is in this sequence.


MATHEMATICA

Select[Range[1000], Length[FactorInteger[# + 1]] == 2 && Length[FactorInteger[#  1]] == 2 &]


PROG

(Python)
from sympy import factorint
def isA007774(n): return len(factorint(n)) == 2
def ok(n): return isA007774(n1) and isA007774(n+1)


CROSSREFS



KEYWORD

nonn


AUTHOR



STATUS

approved



