A157484 Numbers k such that k-+1 are divisible by exactly 4 primes, counted with multiplicity.

55, 89, 151, 197, 233, 249, 295, 307, 329, 341, 343, 349, 461, 489, 491, 569, 571, 665, 713, 739, 775, 851, 857, 859, 869, 871, 949, 1013, 1015, 1061, 1097, 1111, 1149, 1191, 1205, 1207, 1209, 1211, 1219, 1255, 1275, 1277, 1291, 1303, 1315, 1421, 1431, 1449, 1483

Offset

1,1

Links

Harvey P. Dale, Table of n, a(n) for n = 1..2000

Example

55 is a term: 55-1 = 54 = 2*3*3*3 and 55+1 = 56 = 2*2*2*7.

Mathematica

q=4; lst={}; Do[If[Plus@@Last/@FactorInteger[n-1]==q&&Plus@@Last/@FactorInteger[n+1]==q, AppendTo[lst, n]], {n, 7!}]; lst
SequencePosition[PrimeOmega[Range[1200]], {4, _, 4}][[All, 1]]+1 (* Requires Mathematica version 10 or later *) (* Harvey P. Dale, May 08 2019 *)

Prog

(PARI) is(k) = bigomega(k-1)==4 && bigomega(k+1)==4; \\ Jinyuan Wang, Mar 22 2020

Crossrefs

Cf. A014612, A124936, A157483.

Sequence in context: A366848 A320507 A039533 * A230125 A027865 A247681

Adjacent sequences: A157481 A157482 A157483 * A157485 A157486 A157487

Keyword

nonn

Author

Vladimir Joseph Stephan Orlovsky, Mar 01 2009

Status

approved