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A307218 Numbers x with k digits in base 2 (MSD(x)_2 = d_1, LSD(x)_2 = d_k) that are equal to the product of the positions of 1's (see examples and formula). 1
1, 1350, 47520, 1995840, 59376240 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

If 0's instead of 1's are considered, then the sequence starts with 1, 2, 12, 504, ...

If the positions are counted with MSD(x)_2 = d_k and LSD(x)_2 = d_1 then the sequence starts with 1, 2, 6, 12, 576000, ...

Next value greater than 10^12. - Giovanni Resta, Mar 29 2019

LINKS

Table of n, a(n) for n=1..5.

FORMULA

Solutions of the equation x = Product_{j=1..k} ((1/2)*(1+j+(-1)^d_j*(1-j))), where d_j are the digits of x in base 2.

EXAMPLE

1350 in base 2 is 10101000110. The 1's are in positions 1, 3, 5, 9, 10 and 1*3*5*9*10 = 1350.

47520 in base 2 is 1011100110100000. The 1's are in positions 1, 3, 4, 5, 8, 9, 11 and 1*3*4*5*8*9*11 = 47520.

1995840 in base 2 is 111100111010001000000. The 1's are in positions 1, 2, 3, 4, 7, 8, 9, 11, 15 and 1*2*3*4*7*8*9*11*15 = 1995840.

MAPLE

P:=proc(q) local a, b, k, n; for n from 1 to q do

a:=convert(n, base, 2); b:=1; for k from 1 to nops(a) do

if a[k]=1 then b:=b*(nops(a)-k+1); fi; od; if b=n then print(n);

fi; od; end: P(10^9);

PROG

(PARI) b(n)=fromdigits(binary(n), 10); \\ A007088

is(n)={k=1; v=digits(b(n)); for(j=2, #v, if(v[j]==1, k=k*j)); k==n; } \\ Jinyuan Wang, Mar 29 2019

CROSSREFS

Cf. A000030, A007088, A010879, A306286.

Sequence in context: A307586 A224743 A260542 * A278383 A174638 A154375

Adjacent sequences:  A307215 A307216 A307217 * A307219 A307220 A307221

KEYWORD

nonn,base,more

AUTHOR

Paolo P. Lava, Mar 29 2019

STATUS

approved

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Last modified June 12 16:18 EDT 2021. Contains 344959 sequences. (Running on oeis4.)