A053318 a(n) contains n digits (either '2' or '7') and is divisible by 2^n.

2, 72, 272, 2272, 22272, 222272, 7222272, 27222272, 727222272, 2727222272, 72727222272, 772727222272, 7772727222272, 77772727222272, 277772727222272, 2277772727222272, 72277772727222272, 272277772727222272

Offset

1,1

Links

Robert Israel, Table of n, a(n) for n = 1..999

Formula

a(n) = a(n-1) + 10^(n-1)*(2 + 5*(a(n-1)/2^(n-1) mod 2)), i.e., a(n) ends with a(n-1); if a(n-1) is divisible by 2^n then a(n) begins with a 2, if not then a(n) begins with a 7.

Maple

A[1]:= 2:
for n from 2 to 100 do
   if A[n-1] mod 2^n = 0 then A[n]:= A[n-1]+2*10^(n-1)
   else A[n]:= A[n-1]+7*10^(n-1)
     fi
od:
seq(A[i], i=1..100); # Robert Israel, Oct 27 2019

Crossrefs

Cf. A023399, A050621, A050622, A035014.

Sequence in context: A361688 A185120 A217842 * A272757 A187707 A163274

Adjacent sequences: A053315 A053316 A053317 * A053319 A053320 A053321

Keyword

base,nonn

Author

Henry Bottomley, Mar 06 2000

Extensions

Formula corrected by Robert Israel, Oct 27 2019

Status

approved