

A189982


Numbers with prime factorization pqrs^2.


6



420, 630, 660, 780, 924, 990, 1020, 1050, 1092, 1140, 1170, 1380, 1386, 1428, 1470, 1530, 1540, 1596, 1638, 1650, 1710, 1716, 1740, 1820, 1860, 1932, 1950, 2070, 2142, 2220, 2244, 2380, 2394, 2436, 2460, 2508, 2550, 2574, 2580, 2604, 2610, 2652, 2660, 2790
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OFFSET

1,1


COMMENTS

Theorem 4 in GoldstonGrahamPintzYildirim proves that a(n) = a(n+1) for infinitely many n.  Charles R Greathouse IV, Jul 17 2015


LINKS

T. D. Noe, Table of n, a(n) for n = 1..1000
D. A. Goldston, S. W. Graham, J. Pintz, and C. Y. Yıldırım, Small gaps between almost primes, the parity problem, and some conjectures of Erdos on consecutive integers (2008)
Will Nicholes, Prime Signatures
Index to sequences related to prime signature


MATHEMATICA

f[n_]:=Sort[Last/@FactorInteger[n]]=={1, 1, 1, 2}; Select[Range[4000], f]


PROG

(PARI) is(n)=vecsort(factor(n)[, 2])==[1, 1, 1, 2]~ \\ Charles R Greathouse IV, Jul 17 2015


CROSSREFS

Cf. A178739, A189975.
Sequence in context: A069064 A024410 A200521 * A070237 A305416 A156687
Adjacent sequences: A189979 A189980 A189981 * A189983 A189984 A189985


KEYWORD

nonn


AUTHOR

Vladimir Joseph Stephan Orlovsky, May 03 2011


STATUS

approved



