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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 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,1
COMMENTS
Number k such that both k-1 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(n-1) and isA007774(n+1)
print([k for k in range(200) if ok(k)]) # Michael S. Branicky, Sep 10 2022
CROSSREFS
Cf. A007774.
Sequence in context: A306926 A205707 A136491 * A268487 A216687 A005360
KEYWORD
nonn
AUTHOR
Tanya Khovanova, Sep 10 2022
STATUS
approved

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Last modified June 21 03:54 EDT 2024. Contains 373540 sequences. (Running on oeis4.)