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A105149 Number of even semiprimes k such that n^2 < k <= (n+1)^2. 2
0, 1, 1, 2, 1, 2, 2, 2, 1, 3, 2, 3, 3, 2, 4, 2, 3, 3, 4, 5, 1, 6, 3, 5, 3, 4, 4, 5, 4, 6, 5, 5, 3, 6, 5, 7, 6, 4, 6, 5, 7, 6, 5, 6, 6, 8, 8, 5, 6, 8, 7, 6, 5, 9, 9, 7, 10, 6, 7, 8, 5, 10, 6, 10, 9, 8, 8, 10, 8, 11, 5, 9, 9, 13, 10, 9, 9, 9, 8, 8, 10, 12, 7, 11, 12, 12, 10, 10, 12, 10, 12, 10, 10, 10, 11 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,4

COMMENTS

a(n)>=1 because there is always a number 2*prime(i) between n^2 and (n+1)^2 for n>0.

LINKS

Robert Israel, Table of n, a(n) for n = 0..9999

EXAMPLE

a(6)=2 because between 5^2 and 6^2 there are two 2*prime(i): 2*prime(6)=2*13 and 2*prime(7)=2*17.

MAPLE

L:= map(numtheory:-pi, [seq(floor(n^2/2), n=0..100)]):

L[2..-1]-L[1..-2]; # Robert Israel, Feb 04 2018

MATHEMATICA

f[n_] := PrimePi[Floor[n^2/2]]; Table[f[(n + 1)] - f[n], {n, 0, 100}]

CROSSREFS

Cf. A105148.

Sequence in context: A225843 A001227 A060764 * A295894 A068307 A158946

Adjacent sequences:  A105146 A105147 A105148 * A105150 A105151 A105152

KEYWORD

easy,nonn

AUTHOR

Giovanni Teofilatto, Apr 10 2005

EXTENSIONS

Edited and extended by Ray Chandler, Apr 16 2005

STATUS

approved

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Last modified February 22 19:36 EST 2018. Contains 299469 sequences. (Running on oeis4.)