n : 26 = 13 ( rest = nr.impar < 26)
=> 26*13+ (resturile impare)=338+ (1; 3; 5; ...; 25)
n = {26*13+1=339; 341 ; 343 ; ... ; 363}
S₁₃ = 339 + 341 + 343 + ... + 363 =
=338+1 + 338+3 + 338+5 + ... + 338+25 =
= 13*338+ (1+3+5+...+25) = 13*338+ 13² = 4394+169 = 4563 .