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A048639 Binary encoding of A006881, numbers with two distinct prime divisors. 8
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; text; internal format)
OFFSET

1,1

LINKS

Michael De Vlieger, Table of n, a(n) for n = 1..10000

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

MATHEMATICA

Total[2^PrimePi@ # &@ (Map[First, FactorInteger@ #] - 1)] & /@ Select[Range@ 160, SquareFreeQ@ # && PrimeOmega@ # == 2 &] (* Michael De Vlieger, Oct 01 2015 *)

PROG

(PARI) lista(nn) = {for (n=1, nn, if (issquarefree(n) && bigomega(n)==2, f = factor(n); x = sum(k=1, #f~, 2^(primepi(f[k, 1])-1)); print1(x, ", "); ); ); } \\ Michel Marcus, Oct 01 2015

CROSSREFS

Permutation of A018900. Cf. A048640, A048623.

Sequence in context: A286467 A231736 A212437 * A021282 A016613 A079427

Adjacent sequences:  A048636 A048637 A048638 * A048640 A048641 A048642

KEYWORD

nonn

AUTHOR

Antti Karttunen, Jul 14 1999

STATUS

approved

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Last modified October 13 19:35 EDT 2019. Contains 327981 sequences. (Running on oeis4.)