A038481 Sums of 2 distinct powers of 7.

8, 50, 56, 344, 350, 392, 2402, 2408, 2450, 2744, 16808, 16814, 16856, 17150, 19208, 117650, 117656, 117698, 117992, 120050, 134456, 823544, 823550, 823592, 823886, 825944, 840350, 941192, 5764802, 5764808, 5764850, 5765144, 5767202

Offset: 0

Author

Olivier Gérard

Keywords

• nonn
• easy

Links

• Robert Israel, Table of n, a(n) for n = 0..10010

Crossrefs

Base 7 interpretation of A038444.

Programs

Maple

seq(seq(7^i+7^j,j=0..i-1),i=1..10); # Robert Israel, Feb 09 2018

Mathematica

Sort[Total/@Subsets[7^Range[0,10],{2}]] (* Harvey P. Dale, Jul 29 2015 *)

Python

from math import isqrt
def A038481(n): return 7**(m:=isqrt(n<<3)+1>>1)+7**(n-1-(m*(m-1)>>1)) # Chai Wah Wu, Apr 05 2025

Formula

G.f.: ((1-x)*(1-7*x))^(-1) * (2 - 8*x + sum_{k>=0} ((1+5*7^k)*x^(k*(k+1)/2) + (1+41*7^k)*x^(1+k*(k+1)/2))). - Robert Israel, Feb 09 2018