A309809 a(n) is the concatenation of n and 2n+1.
13, 25, 37, 49, 511, 613, 715, 817, 919, 1021, 1123, 1225, 1327, 1429, 1531, 1633, 1735, 1837, 1939, 2041, 2143, 2245, 2347, 2449, 2551, 2653, 2755, 2857, 2959, 3061, 3163, 3265, 3367, 3469, 3571, 3673, 3775, 3877, 3979, 4081, 4183, 4285, 4387, 4489, 4591, 4693, 4795, 4897
Offset: 1
Examples
a(3)=37 is the concatenation of 3 and 7.
Links
- Robert Israel, Table of n, a(n) for n = 1..10000
Programs
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Maple
seq(n*10^(ceil(log[10](2*n+1)))+2*n+1, n=1..100);
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Mathematica
Table[FromDigits[Join[IntegerDigits[n],IntegerDigits[2n+1]]],{n,50}] (* Harvey P. Dale, Jan 30 2024 *)
Formula
a(n) = n*10^(ceiling(log_10(2*n+1))) + 2*n + 1.