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A137905 Numbers that appear as binomial coefficients exactly twice. 4
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 (list; graph; refs; listen; history; text; internal format)
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

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Last modified September 24 09:10 EDT 2021. Contains 347630 sequences. (Running on oeis4.)