A048639 - OEIS

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A048639

Binary encoding of A006881, numbers with two distinct prime divisors.

3, 5, 9, 6, 10, 17, 33, 18, 65, 12, 129, 34, 257, 66, 20, 130, 513, 1025, 36, 258, 2049, 24, 4097, 68, 8193, 514, 40, 1026, 16385, 132, 32769, 2050, 260, 65537, 72, 131073, 4098, 8194, 136, 262145, 16386, 524289, 48, 516, 1048577, 1028, 2097153, 32770 (list; graph; refs; listen; history; internal format)

	OFFSET	`1,1`
	FORMULA	`a(n) = 2^(i-1)+2^(j-1), where A006881[ n ] = p_i*p_j (p_i and p_j stand for the i-th and j-th primes respectively, where the first prime is 2).`
	MAPLE	`encode_A006881 := proc(upto_n) local b, i; b := [ ]; for i from 1 to upto_n do if((0 <> mobius(i)) and (4 = tau(i))) then b := [ op(b), bef(i) ]; fi; od: RETURN(b); end; # see A048623 for bef`
	CROSSREFS	`Permutation of A018900. Cf. A048640, A048623.` `Sequence in context: A199612 A076844 A198558 * A021282 A016613 A079427` `Adjacent sequences: A048636 A048637 A048638 * A048640 A048641 A048642`
	KEYWORD	`nonn`
	AUTHOR	`Antti Karttunen, Jul 14 1999`

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Last modified February 15 07:22 EST 2012. Contains 205706 sequences.