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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 (list; graph; refs; listen; history; text; internal format)
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

Cf. A000040, A001358, A062721, A077068.

Sequence in context: A283325 A318438 A294233 * A079087 A324920 A236855

Adjacent sequences:  A121881 A121882 A121883 * A121885 A121886 A121887

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

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Last modified September 18 13:54 EDT 2021. Contains 347527 sequences. (Running on oeis4.)