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A080393 a(n) is the smallest integer such that A080383(a(n)) = n. 1
0, 1, 2, 3, 40, 5, 12, 25, 1122, 13, 420, 1103, 12324, 421, 4960, 11289, 232582, 4961, 16356, 107073 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,3

COMMENTS

Parity of n and a(n) is opposite.

It is unknown whether all positive integers arise in A080383 or not.

a(22)=16357, a(23)=90306, a(26)=90307. - Vaclav Kotesovec, Sep 10 2019

For each n > 20 except 22, 23, and 26, a(n) > 10^6 (if it exists). - Jon E. Schoenfield, Sep 15 2019

LINKS

Table of n, a(n) for n=1..20.

EXAMPLE

a(10)=13 because in A080383 10 appears first as the 13th term.

MATHEMATICA

f[x_] := Count[Table[IntegerQ[Binomial[x, Floor[x/2]]/ Binomial[x, j]], {j, 0, n}], True]; t=Table[0, {20}]; Do[s=f[n]; If[s<21&&t[[s]]==0, t[[s]]=n], {n, 1, 1300}]; t

PROG

(PARI) f(n) = my(b=binomial(n, n\2)); sum(i=0, n, (b % binomial(n, i)) == 0); \\ A080383

a(n) = my(k=0); while(f(k) != n, k++); k; \\ Michel Marcus, Aug 23 2019

CROSSREFS

Cf. A080383, A080384(1)=a(6), A080385(1)=a(7), A080386(1)=a(8), A080387(1)=a(10).

Sequence in context: A331204 A153745 A076724 * A111683 A323734 A347817

Adjacent sequences: A080390 A080391 A080392 * A080394 A080395 A080396

KEYWORD

nonn,more

AUTHOR

Labos Elemer, Mar 17 2003

EXTENSIONS

a(13)-a(16) from Michel Marcus, Aug 23 2019

a(17) from Jon E. Schoenfield, Sep 15 2019

a(18) from Michel Marcus, Aug 23 2019

a(19) from Vaclav Kotesovec, Sep 10 2019

a(20) from Jon E. Schoenfield, Sep 15 2019

STATUS

approved

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Last modified December 4 19:07 EST 2022. Contains 358563 sequences. (Running on oeis4.)