A137905 Numbers that appear as binomial coefficients exactly twice.

3, 4, 5, 7, 8, 9, 11, 12, 13, 14, 16, 17, 18, 19, 22, 23, 24, 25, 26, 27, 29, 30, 31, 32, 33, 34, 37, 38, 39, 40, 41, 42, 43, 44, 46, 47, 48, 49, 50, 51, 52, 53, 54, 57, 58, 59, 60, 61, 62, 63, 64, 65, 67, 68, 69, 71, 72, 73, 74, 75, 76, 77, 79, 80, 81, 82, 83, 85, 86, 87, 88

Offset

1,1

Comments

Complement of A006987; a(n) = A058084(a(n)). - Reinhard Zumkeller, Mar 20 2009

Links

Table of n, a(n) for n=1..71.

Formula

a(n) = A185024(n+1). - Elijah Beregovsky, May 14 2019

Example

7 is a member because 7 = binomial(7, 1) = binomial(7, 6) and no other binomial coefficient equals 7. [clarified by Jonathan Sondow, Jan 12 2018]

Prog

(PARI) isok(n) = (sum(i=0, n, sum(j=0, i, binomial(i, j)==n)) == 2) \\ Michel Marcus, Jun 16 2013

Crossrefs

Cf. A006987, A058084, A062527, A098564, A098566.

Sequence in context: A183574 A184425 A276212 * A285958 A179779 A284679

Adjacent sequences: A137902 A137903 A137904 * A137906 A137907 A137908

Keyword

easy,nonn

Author

David Wasserman, Feb 21 2008

Status

approved