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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 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Theorem 4 in Goldston-Graham-Pintz-Yildirim 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

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Last modified January 17 14:12 EST 2019. Contains 319225 sequences. (Running on oeis4.)