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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.
8
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
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
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: A024410 A200521 A350374 * A376374 A070237 A305416
KEYWORD
nonn
AUTHOR
EXTENSIONS
Definition reworded by M. F. Hasler, Jul 17 2019
STATUS
approved