we have
a0=0
a(i+1)=2ai+1
Verify
For i=0
a(0+1)=2(0)+1
a(1)=1
For i=1
a(1+1)=2(1)+1
a(2)=3
For i=2
a(2+1)=2(3)+1
a(3)=7
therefore
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