29 U.S.C. § 1363: Liability of substantial employer for withdrawal from single-employer plans under multiple controlled groups

shall notify the corporation of the withdrawal during a plan year of a substantial employer for such plan year from the plan, within 60 days after such withdrawal, and request that the corporation determine the liability of all persons with respect to the withdrawal of the substantial employer. the numerator of which is the total amount required to be contributed to the plan by such contributing sponsors for the last 5 years ending prior to the withdrawal, and the denominator of which is the total amount required to be contributed to the plan by all contributing sponsors for such last 5 years. In lieu of payment of a contributing sponsor’s liability under this section, the contributing sponsor may be required to furnish a bond to the corporation in an amount not exceeding 150 percent of his liability to insure payment of his liability under this section. The bond shall have as surety thereon a corporate surety company which is an acceptable surety on Federal bonds under authority granted by the Secretary of the Treasury under sections 9304–9308 of title 31. Any such bond shall be in a form or of a type approved by the Secretary including individual bonds or schedule or blanket forms of bonds which cover a group or class. If the plan is not terminated under section 1341(c) or 1342 of this title within the 5-year period commencing on the day of withdrawal, the liability is abated and any payment held in escrow shall be refunded without interest (or the bond cancelled) in accordance with bylaws or rules prescribed by the corporation. demand payment or realize on the bond and hold such amount in escrow for the benefit of the plan; treat any escrowed payments under this section as if they were plan assets and apply them in a manner consistent with this subtitle; and refund any amount to the contributing sponsor which is not required to meet any obligation of the corporation with respect to the plan. require the plan fund to be equitably allocated between those participants no longer working in covered service under the plan as a result of the withdrawal, and those participants who remain in covered service under the plan; treat that portion of the plan funds allocable under paragraph (1) to participants no longer in covered service as a plan terminated under section 1342 of this title ; and treat that portion of the plan fund allocable to participants remaining in covered service as a separate plan. The corporation is authorized to waive the application of the provisions of subsections (b), (c), and (d) of this section whenever it determines that there is an indemnity agreement in effect among contributing sponsors under the plan which is adequate to satisfy the purposes of this section and of section 1364 of this title .