Step 1: Take a natural number n 1=4 and calculate n? + 1A 1 = (5)

Step 2: Calculate the sum of digits of a 1 to get n2, and calculate n2? +1 a2a 2 =(65)

Step 3: Calculate the sum of digits of a2 to get n3, and then calculate n3? A3a3 of+1 = (112)

......

And so on, a20 12=(65)

Analysis:

Judging from the meaning of the question:

n 1=4+ 1=5，a 1 = 5×5+ 1 = 26；

n2=2+6=8，a2 = 8×8+ 1 = 65；

n3=6+5= 1 1，a3 = 1 1× 1 1+ 1 = 122；

n4= 1+2+2=5，a4 = 5×5+ 1 = 26；

…

Three-number cycle

∴n20 12 is the second in the 672nd cycle.

∴a20 12=a 1=65.