In Jainism, particularly among Shvetambara lay followers, taking tivihar pachkhan (three-fold vow of restraint after meals) is a recommended practice but not always considered absolutely mandatory after ekashana (taking food once a day).

Purpose of Tivihar Pachkhan: Tivihar pachkhan typically involves making a vow not to eat, drink, or consume anything after the meal until the next day’s meal. This is done to observe greater self-restraint, minimize himsa (violence), and maintain discipline.

After Ekashana:

• It is a good practice to take tivihar pachkhan immediately after completing ekashana.
• This helps in setting a clear intention and spiritual boundary, reinforcing the principle of aparigraha (non-possessiveness) and self-control.
• Many Jain texts and learned teachers recommend this for laypeople (shravaks and shravikas) to avoid accidental consumption and to sanctify the vow.

Necessity:

• While it is highly encouraged and considered ideal, it is not a strict doctrinal requirement for all. Some may follow the practice informally, especially if they already have the strong resolve not to consume anything further.
• However, for those wishing to observe the vow in its fullest spiritual and ritual sense, taking the pachkhan right after ekashana is advised.

In Summary: It is not strictly necessary, but it is highly recommended and spiritually beneficial to take tivihar pachkhan right after doing ekashana according to Jain tradition.