a(n) is the number of words of length n over an alphabet of size 4 that are in standard order and which have the property that every letter that appears in the word is repeated.
A278989
a(n) is the number of words of length n over an alphabet of size 4 that are in standard order and which have the property that every letter that appears in the word is repeated.
Terms
- a(0) =0a(1) =0a(2) =1a(3) =1a(4) =4a(5) =11a(6) =41a(7) =162a(8) =715a(9) =3425a(10) =16777a(11) =80928a(12) =379347a(13) =1726375a(14) =7654817a(15) =33219630a(16) =141692075a(17) =596122477a(18) =2480969257a(19) =10237751324a(20) =41963944275a(21) =171103765747a(22) =694775280993
External references
- oeis: A278989