semiprime
A composite number^{} which is the product of two (possibly equal) primes is called semiprime. Such numbers are sometimes also called 2almost primes. For example:

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1 is not a semiprime because it is not a composite number or a prime,

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2 is not a semiprime, as it is a prime,

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4 is a semiprime, since $4=2\cdot 2$,

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8 is not a semiprime, since it is a product of three primes ($8=2\cdot 2\cdot 2$),

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2003 is not a semiprime, as it is a prime,

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2005 is a semiprime, since $2005=5\cdot 401$,

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2007 is not a semiprime, since it is a product of three primes ($2007=3\cdot 3\cdot 223$).
The first few semiprimes are $4,6,9,10,14,15,21,22,25,26,33,34,35,38,39,46,49,51,55,57,58,62,\mathrm{\dots}$ (http://www.research.att.com/cgibin/access.cgi/as/njas/sequences/eisA.cgi?Anum=001358Sloane’s sequence A001358 ). The Moebius function $\mu (n)$ for semiprimes can be only equal to 0 or 1. If we form an integer sequence of values of $\mu (n)$ for semiprimes we get a binary sequence: $0,1,0,1,1,1,1,1,0,1,1,1,1,1,1,1,0,1,1,1,1,1,\mathrm{\dots}$. (http://www.research.att.com/cgibin/access.cgi/as/njas/sequences/eisA.cgi?Anum=072165Sloane’s sequence A072165 ).
All the squares of primes are also semiprimes. The first few squares of primes are then $4,9,25,49,121,169,289,361,529,841,961,1369,1681,1849,2209,2809,3481,3721,4489,5041,\mathrm{\dots}$. (http://www.research.att.com/cgibin/access.cgi/as/njas/sequences/eisA.cgi?Anum=001248Sloane’s sequence A001248 ). The Moebius function $\mu (n)$ for the squares of primes is always equal to 0 as it is equal to 0 for all squares.
Title  semiprime 

Canonical name  Semiprime 
Date of creation  20130322 12:49:22 
Last modified on  20130322 12:49:22 
Owner  drini (3) 
Last modified by  drini (3) 
Numerical id  9 
Author  drini (3) 
Entry type  Definition 
Classification  msc 11A41 
Synonym  semiprime 
Synonym  2almost prime 
Defines  almost prime 