The angel number 421:

Select any whole number. If it is an even number, divide by 2; if it is odd number multiply by 3 and add 1. Repeat the process until you get the loop value 4, 2, 1 in repetition. Example:

Whole number is 15.

15 is an odd no; so (15 × 3) + 1 = 46

46 is an even no; so 46 / 2 = 23

23 is an odd no; so (23 × 3) + 1 = 70

70 is an even no; so 70 / 2 = 35

35 is an odd no; so (35 × 3) + 1 = 106

106 is an even no; so 106 / 2 = 53

53 is an odd no; so (53 × 3) + 1 = 160

160 is an even no; so 160 / 2 = 80

80 is an even no; so 80 / 2 = 40

40 is an even no; so 40 / 2 = 20

20 is an even no; so 20 / 2 = 10

10 is an even no; so 10 / 2 = 5

5 is an odd no; so (5 × 3) + 1 = 16

16 is an even no; so 16 / 2 = 8

8 is an even no; so 8 / 2 = 4

4 is an even no; so 4 / 2 = 2

2 is an even no; so 2 / 2 = 1

1 is an odd no; so (1 × 3) + 1 = 4

4 is an even no; so 4 / 2 = 2

2 is an even no; so 2 / 2 = 1

So the loop 4..2..1 goes on and on.