A261077 - OEIS

The OEIS is supported by the many generous donors to the OEIS Foundation.

Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 61st year, we have over 378,000 sequences, and we’ve reached 11,000 citations (which often say “discovered thanks to the OEIS”).

Other ways to Give

A261077

Semiprimes whose prime factors differ from each other in one bit position only.

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

(list; graph; refs; listen; history; text; internal format)

OFFSET

1,1

LINKS

Antti Karttunen, Table of n, a(n) for n = 1..5000

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. A000523, A001222, A006530, A020639, A101080.

Cf. also A261073, A261080 (subsequences).

Subsequence of A261078.

Gives the positions of ones in A260737.