In 1932, Hayek suggested that private investment in the public markets was a better road to wealth and economic co-ordination in Britain than government spending programs, as argued in a letter he co-signed with Lionel Robbins and others in an exchange of letters with John Maynard Keynes in The Times. The nearly decade long deflationary depression in Britain dating from Churchill's decision in 1925 to return Britain to the gold standard at the old pre-war, pre-inflationary par was the public policy backdrop for Hayek's single public engagement with Keynes over British monetary and fiscal policy, otherwise Hayek and Keynes agreed on many theoretical matters, and their economic disagreements were fundamentally theoretical, having to do almost exclusively with the relation of the economics of extending the length of production to the economics of labour inputs.
Try to answer this question if possible (otherwise reply "unanswerable"): What other notable figure signed the letter in which Hayek made his statement regarding private investment?
Lionel Robbins