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 A189982 Numbers with prime signature (2,1,1,1), i.e., factorization p*q*r*s^2 with distinct primes p, q, r, s. 7
 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+1) = a(n) + 1 for infinitely many n. - Charles R Greathouse IV, Jul 17 2015, corrected by M. F. Hasler, Jul 17 2019 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 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 Part of the list A178739 .. A179696 and A030514 .. A030629, A189975 .. A189990 etc., cf. A101296. 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 EXTENSIONS Definition reworded by M. F. Hasler, Jul 17 2019 STATUS approved

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Last modified May 24 19:14 EDT 2020. Contains 334580 sequences. (Running on oeis4.)