A357074 Numbers sandwiched between a pair of numbers each with exactly two prime factors (counted without multiplicity).

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

Offset

1,1

Comments

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

Links

Table of n, a(n) for n=1..59.

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

Adjacent sequences: A357071 A357072 A357073 * A357075 A357076 A357077

Keyword

nonn

Author

Tanya Khovanova, Sep 10 2022

Status

approved