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A333747
Numbers that are either the product of two consecutive primes or two primes with a prime in between.
3
6, 10, 15, 21, 35, 55, 77, 91, 143, 187, 221, 247, 323, 391, 437, 551, 667, 713, 899, 1073, 1147, 1271, 1517, 1591, 1763, 1927, 2021, 2279, 2491, 2773, 3127, 3233, 3599, 3953, 4087, 4331, 4757, 4891, 5183, 5609, 5767, 6059, 6557, 7031, 7387, 8051, 8633, 8989, 9797, 9991
OFFSET
1,1
COMMENTS
In other words, these are numbers that are the product of two distinct primes whose prime indices differ by at most two.
LINKS
FORMULA
Union of A006094 and A090076.
a(n) = prime(ceiling(n/2))*prime(ceiling((n+3)/2)).
a(2*n-1) = prime(n)*prime(n+1).
a(2*n) = prime(n)*prime(n+2).
MAPLE
R:= NULL;
p:= 2; q:= 3;
for n from 1 to 100 by 2 do
r:= nextprime(q);
R:= R, p*q, p*r;
p:= q; q:= r;
od:
R; # Robert Israel, Apr 22 2020
MATHEMATICA
a[n_] := Prime[Ceiling[n/2]] * Prime[Ceiling[(n + 3)/2]]; Array[a, 50] (* Amiram Eldar, Apr 04 2020 *)
CROSSREFS
Subsequence of A001358.
Sequence in context: A211007 A315279 A122783 * A124000 A229321 A229323
KEYWORD
nonn,easy
AUTHOR
Bobby Jacobs, Apr 03 2020
STATUS
approved