A343586 a(n) = the sum of all the multiples of 2 or 5 less than or equal to 10^n.
35, 3050, 300500, 30005000, 3000050000, 300000500000, 30000005000000, 3000000050000000, 300000000500000000, 30000000005000000000, 3000000000050000000000, 300000000000500000000000, 30000000000005000000000000, 3000000000000050000000000000
Offset: 1
Examples
a(1) = 2 + 5 + 4 + 6 + 8 + 10 = 3*10^(2*1-1) + 5*10^(1-1) = 35.
Links
- Nicolo Sartori di Borgoricco, Proof that formula is correct
- Index entries for linear recurrences with constant coefficients, signature (110,-1000).
Crossrefs
Cf. A065502 (multiples of 2 or 5).
Formula
a(n) = 3*10^(2*n-1) + 5*10^(n-1).
G.f.: 5*x*(7 - 160*x)/(1 - 110*x + 1000*x^2). - Stefano Spezia, Apr 21 2021
a(n) = 110*a(n-1)-1000*a(n-2). - Wesley Ivan Hurt, Apr 26 2021