The sum of digits is a triangular number of 1, and its ordinal sum is only 1, 4, 7.
A triangle whose ordinal sum is 1 minus 1 divided by 9 is still a triangle, and its ordinal sum is comprehensive.
Is the sum of ordinal numbers a triangle of 4 minus 1 divided by 9 or a triangle of numbers and 1?
The sum of ordinal digits is the triangle number of 7 minus 1 divided by 9, and the sum of digits is the triangle number of 3, 6 and 9.
The sum of digits is a triangular number of 3, and the sum of ordinal numbers is only 2 and 6.
The sum of ordinal numbers is the triangle number of 2 plus 6 divided by 9, and the whole number minus ordinal numbers divided by 9 (rounding, the same below) is still the triangle number of numbers and 3, 6 and 9.
The sum of ordinal numbers is the triangle number of 6, minus 3 divided by 9, plus all ordinal numbers divided by 9, it is still the triangle number of numbers and 3, 6 and 9.
The sum of digits is a triangle of 6, and the sum of ordinal numbers is only 3 and 5.
The sum of ordinal numbers is a triangular number of 3, minus 6 divided by 9, plus all ordinal numbers divided by 9, or a number and the triangular number of 1.
The sum of ordinal numbers is a triangular number of 5, plus 3 divided by 9, and all the numbers minus ordinal numbers divided by 9 are still a number and the triangular number of 1.
Mathematicians' achievements in this respect:
Mathematicians in China have long known about arithmetic progression. In China's earliest mathematical work "Weekly Calculations", it is said that the diameter of the "seven scales" (the circumference of the sun and the moon) increases 19833+000 steps ×2, which is arithmetic progression.
China's important mathematical work, Nine Chapters Arithmetic, was written about 1 century, but there are some problems in the two chapters of descent and equal loss related to arithmetic progression.
At the end of the 5th century, Zhang Qiujian in the Southern and Northern Dynasties wrote Zhang Qiujian's computational classics, including three questions about arithmetic progression.