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

Author

Vladimir Joseph Stephan Orlovsky, Mar 01 2009

Keywords

• nonn

Examples

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

Links

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

Crossrefs

Cf. A014612, A124936, A157483.

Programs

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 *)

PARI

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