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is a strictly stationary random field, with X 0 bounded and expressible as a sum of indicator functions satisfying certain conditions, if the mixing coefficient :(s) is summable over Z d (that, is, m m d&1 :(m)< ), and if a mixing condition involving three sets is satisfied, then the third order cumulant Cum(X a , X b , X c ) of X has a continuous spectral density. We do not begin with the assumption that the cumulants are absolutely summable. Academic PressAMS 1991 subject classifications: 60.

Malignant gastrointestinal neuroectodermal tumour (GNET) is a rare, aggressive neoplasm with fewer than 100 cases reported in the literature. Most cases arise in the tubular gastrointestinal tract. We reported a unique case of GNET arising in the extrahepatic bile ducts and reviewed the literature of GNETs. The patient is a female in her mid-30s who presented with painless jaundice and diarrhoea several months after cholecystectomy for biliary dyskinesia. Workup revealed a tumour arising from the peripheral 4B bile ducts involving the left hepatic duct and bifurcation. Histologic examination of the lesion showed a malignant spindled and epithelioid neoplasm which strongly expressed S100 and SOX-10. Neoplastic cells were negative for various cytokeratins and melanoma markers. FISH testing using EWSR1 break-apart probes showed rearrangement of the EWSR1 gene region. The immunohistochemical and molecular findings are consistent with a diagnosis of GNET arising in the extrahepatic bile ducts.