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A179696
Numbers with prime signature {7,1,1}, i.e., of form p^7*q*r with p, q and r prime.
6
1920, 2688, 4224, 4480, 4992, 6528, 7040, 7296, 8320, 8832, 9856, 10880, 11136, 11648, 11904, 12160, 14208, 14720, 15232, 15744, 16512, 17024, 18048, 18304, 18560, 19840, 20352, 20608, 21870, 22656, 23424, 23680, 23936, 25728, 25984, 26240, 26752, 27264
OFFSET
1,1
MAPLE
a:= proc(n) option remember; local k;
for k from 1+ `if` (n=1, 1, a(n-1))
while sort (map (x-> x[2], ifactors(k)[2]), `>`)<>[7, 1, 1]
do od; k
end:
seq (a(n), n=1..40); # Alois P. Heinz, Jan 23 2011
MATHEMATICA
f[n_]:=Sort[Last/@FactorInteger[n]]=={1, 1, 7}; Select[Range[30000], f]
PROG
(PARI) list(lim)=my(v=List(), t1, t2); forprime(p=2, (lim\6)^(1/7), t1=p^7; forprime(q=2, lim\t1, if(p==q, next); t2=t1*q; forprime(r=q+1, lim\t2, if(p==r, next); listput(v, t2*r)))); vecsort(Vec(v)) \\ Charles R Greathouse IV, Jul 20 2011
KEYWORD
nonn
AUTHOR
EXTENSIONS
Title edited by Daniel Forgues, Jan 22 2011
STATUS
approved