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 A101681 Numbers k such that gcd (C(2k,k), 2k+1) > 1. 2
 7, 16, 17, 19, 22, 25, 31, 34, 38, 42, 43, 45, 46, 47, 49, 52, 55, 58, 61, 64, 67, 70, 71, 72, 73, 76, 77, 79, 80, 82, 87, 88, 92, 93, 94, 97, 100, 102, 103, 104, 106, 107, 110, 112, 115, 117, 122, 123, 124, 127, 129, 130, 133, 136, 139, 142, 143, 145, 147, 148 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Positions where A056616 differs from A000984. The set seems to have greater cardinality than its complement. LINKS David A. Corneth, Table of n, a(n) for n = 1..10000 EXAMPLE 7 is in the sequence as gcd(binomial(2*7, 7), 2*7 + 1)) = gcd(3432, 15) = gcd(3*1144, 3*5) > 1. - David A. Corneth, Apr 03 2021 MATHEMATICA Select[Range[200], GCD[Binomial[2 #, #], 2 #+1]>1&] (* Harvey P. Dale, May 11 2019 *) PROG (PARI) is(n) = { my(f = factor(2*n+1)); for(i = 1, #f~, if(val(2*n, f[i, 1])-2*val(n, f[i, 1]) > 0, return(1))); 0 } val(n, p) = my(r=0); while(n, r+=n\=p); r \\ David A. Corneth, Apr 03 2021 CROSSREFS Cf. A000984, A056616, A126786. Sequence in context: A258771 A063593 A070417 * A067776 A193688 A132348 Adjacent sequences: A101678 A101679 A101680 * A101682 A101683 A101684 KEYWORD nonn AUTHOR Ralf Stephan, Dec 11 2004 STATUS approved

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Last modified January 29 23:01 EST 2023. Contains 359939 sequences. (Running on oeis4.)