A048639 - OEIS

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

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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 * A369829 A340593 A021282

Adjacent sequences: A048636 A048637 A048638 * A048640 A048641 A048642

KEYWORD

nonn

AUTHOR

Antti Karttunen, Jul 14 1999

STATUS

approved