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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
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 * A340593 A021282 A016613
KEYWORD
nonn
AUTHOR
Antti Karttunen, Jul 14 1999
STATUS
approved

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Last modified March 28 09:04 EDT 2024. Contains 371240 sequences. (Running on oeis4.)