Olli Lehto's institutional history of the 1978 Helsinki ICM, which covers the discrimination controversy in detail, does not mention the fifteen-signer 'Situation in Soviet Mathematics' memorandum

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Olli Lehto's Mathematics Without Borders: A History of the International Mathematical Union (1998) — already a Tier-1 source in this vault for Pontryagin's conduct at the IMU Executive Committee level (see claim-pontryagin-opposed-margulis-fields-medal-at-imu-executive-committee-paris-1978) — devotes its §9.6 ("ICM-1978 in Helsinki") to exactly the terrain the "Situation in Soviet Mathematics" memorandum would occupy: the low Soviet attendance, Margulis's blocked travel, and an organized human-rights event on discrimination against Jewish mathematicians in the USSR. Lehto, who personally chaired the Congress, writes: "I heard of plans of an important event whose aim was to call attention to cases where human rights had been violated, in particular to the discrimination against Jewish mathematicians in the USSR... The person in charge of the arrangements was Lipman Bers, whom I knew well." Despite this level of detail about the Congress's politics — including describing "the partial boycott on the part of the USSR" (the Soviet side's own travel denial of its delegates) — Lehto's chapter never mentions a fifteen-signer memorandum, a counter-boycott proposal from the American side, or Pontryagin being named as a discrimination target in any such document.

This is a negative finding from a well-positioned primary witness, not proof the memo doesn't exist or doesn't name Pontryagin. Lehto's book is an institutional history of the IMU's own business; a memo circulated informally among American mathematicians, debated internally, and published (if at all) in an American professional-society newsletter sits outside that frame. It means the vault's one direct-read Tier-1 source for this exact Congress offers no corroboration either way, alongside claim-no-source-confirms-1978-icm-memo-names-pontryagin-individually and claim-gerovitch-abstract-frames-1978-icm-memo-discrimination-charge-collectively.