The Tribonacci numbers
Known as: The Tribonacci numbers
The sum of the last three numbers.
The first 37 terms (starting from n=1) are:
1 1 1 3 5 9 17 31 57 105 193 355 653 1201 2209 4063 7473 13745 25281 46499 85525 157305 289329 532159 978793 1800281 3311233 6090307 11201821 20603361 37895489 69700671 128199521 235795681 433695873 797691075 1467182629