login
A261077
Semiprimes whose prime factors differ from each other in one bit position only.
5
6, 21, 33, 35, 57, 65, 161, 185, 201, 323, 377, 393, 437, 473, 497, 713, 899, 1529, 1577, 1763, 1769, 1841, 1961, 2021, 2537, 3233, 3473, 3497, 3737, 4553, 4601, 4757, 5561, 5609, 5753, 6497, 7217, 7313, 9593, 9797, 10265, 10403, 10841, 10961, 11009, 12297, 14129, 15689, 17513, 18209, 19043, 19337, 21353, 22499, 23129, 23393, 26969, 27221, 27233, 29177
OFFSET
1,1
LINKS
EXAMPLE
21 = 3*7 is present because 3 in binary is "11" ("011" when extended with a leading zero) and 7 in binary is "111", and these differ only in the bit-position 2 (with indexing where the least significant bit is in the position 0).
33 = 3*11 is present because 3 in binary is "11" ("0011" when extended with two leading zeros) and 11 in binary is "1011", and these differ only in the bit-position 3.
PROG
(PARI)
A020639(n) = if(1==n, n, vecmin(factor(n)[, 1]));
isA261077 = n -> if(bigomega(n)!=2, 0, (1 == norml2(binary(bitxor((n/A020639(n)), A020639(n))))));
i=0; n=0; while(i < 5000, n++; if(isA261077(n), i++; write("b261077.txt", i, " ", n)));
(Scheme, with Antti Karttunen's IntSeq-library)
(define A261077 (MATCHING-POS 1 1 (lambda (n) (and (= 2 (A001222 n)) (= 1 (A101080bi (A020639 n) (A006530 n)))))))
CROSSREFS
Cf. also A261073, A261080 (subsequences).
Subsequence of A261078.
Gives the positions of ones in A260737.
Sequence in context: A302868 A064440 A106634 * A063126 A256866 A115702
KEYWORD
nonn,base
AUTHOR
Antti Karttunen, Sep 22 2015
STATUS
approved