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A121884
Excess of n-th semiprime over previous prime.
1
1, 1, 2, 3, 1, 2, 2, 3, 2, 3, 2, 3, 4, 1, 2, 3, 2, 4, 2, 4, 5, 1, 4, 2, 1, 4, 3, 2, 3, 4, 2, 4, 5, 6, 3, 2, 2, 5, 6, 8, 9, 10, 2, 2, 3, 2, 3, 4, 6, 7, 4, 1, 2, 4, 3, 2, 4, 5, 2, 4, 6, 1, 2, 3, 4, 6, 7, 10, 2, 3, 4, 6, 7, 8, 10, 3, 2, 4, 6, 8, 2, 3, 2, 5, 2, 4, 3, 1, 4, 6, 8, 2, 5, 6, 8, 9, 10, 12, 2, 1, 2, 4, 6
OFFSET
1,3
COMMENTS
a(n) = 1 iff n is in A077068. a(n) = 2 iff n is in A062721.
LINKS
Robert G. Wilson v, Table of n, a(n) for n=1..1000
FORMULA
a(n) = Min{A001358(n)-p for p < A001358(n) and p in A000040(k)}.
MATHEMATICA
semiPrimeQ[n_] := Plus @@ Last /@ FactorInteger[n] == 2; lst1 = Select[ Range@ 325, semiPrimeQ@# &]; lst = Select[ Range@ 500, semiPrimeQ@# &]; lst - (NextPrime[ #, -1] & /@ lst) (* Robert G. Wilson v, Mar 16 2009 *)
#-NextPrime[#, -1]&/@Select[Range[400], PrimeOmega[#]==2&] (* Harvey P. Dale, Feb 15 2018 *)
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Jonathan Vos Post, Aug 31 2006
EXTENSIONS
a(31)-a(103) from Robert G. Wilson v, Mar 16 2009
STATUS
approved