Rules Each vertical and horizontal row contains one number from 1 to 9. The arrow in the square indicates which side of the square the number before and after that number (if the number in the square is 5, then 4 and 6) is on on either side of the square in that row. If it's on the same side, it's attached to that direction only, and if it's on both sides, it's attached to both. Consecutive numbers The number in the square and the numbers before and after it are consecutive are called "consecutive numbers". Single arrow If two consecutive numbers are both in the same direction as seen from that square, only one arrow will be attached in that direction. This state is called "single arrow". (Example) If both 4 and 6 are to the right of 5, only the right side of 5 will have an arrow as shown below. 　　 Double arrow If there are two consecutive numbers on either side of the square, there will be two arrows in each direction. This state is called "double arrow". (Example) If 4 and 6 are separated to the left and right of 5, arrows are attached to both sides of 5 as shown below. 　　 Way to think There are four basic ideas for solving problems, and most problems can be solved using these four concepts. (1) 1 and 9 cannot be entered in squares with double arrows. (2) Consecutive numbers cannot be entered in squares where the arrows are not facing each other. (3) There are consecutive numbers at the end of the arrow. (4) All numbers are connected by arrows. About (1) The only consecutive digits of 1 are 2, and only consecutive digits of 9 are 8, and both have only one consecutive number. Therefore, 1 and 9 are always single arrows both vertically and horizontally, so they do not enter a square with a double arrow. There are always two or more squares without double arrows in the column, and when there are only two squares, 1 and 9 are entered there. In the figure below, there is a double arrow in the gray square, so 1 and 9 cannot be entered. 　　 About (2) For example, in the case of 5, 4 and 6 are consecutive numbers, but 5 is the consecutive number when viewed from 4 or 6. In other words, 5 and 4, and 5 and 6 are consecutive numbers, so the arrows face each other. Therefore, consecutive numbers go into squares with arrows facing each other, and not into squares with arrows not facing each other. In the figure below, 8 is not included in the gray square. 　　 About (3) It is natural that there is a series of numbers at the end of the arrow, but it is surprisingly easy to overlook. In the figure below, there is an arrow on the left side of 4, but 3 and 5 are not on the left side. Therefore, 3 or 5 will fit in the red square. 　　 About (4) If you follow the arrows in order from 1 in the figure below, you will reach 9, and you can return from 9 to 1 in the same way. When judging whether the numbers are correct or not, the important point is whether or not they are connected to the next.